In the Name of Allah — the Most Beneficent, the Most Merciful.
Stand on a shore at night and look out to sea, and you are looking at most of the world. The ocean covers more of the earth's surface than all the land put together. It holds almost all of the planet's water, it sets the climate of every continent, it feeds billions of people, and most of it has never been seen by any human eye. It is the largest habitat on earth, and also the least explored. This post does what the first two posts in this series did for earthquakes and for astronomy: first the phenomenon as science understands it, then the argument for how it relates to the Qur'anic narrative.
1. What the ocean is
The ocean is a single body of salt water covering about 71 percent of the earth's surface — some 361 million square kilometres. It holds about 1.335 billion cubic kilometres of water, which is about 97 percent of all the water on earth. The remaining few percent is locked in ice caps and glaciers, stored underground, or flowing in rivers and lakes. If all the water of the ocean were spread evenly over the earth, the whole planet would be covered to a depth of about 2.6 kilometres.
It is divided by convention into five oceans. The Pacific is by far the largest, at about 165 million square kilometres — larger than all the land on earth combined. Then come the Atlantic at about 106 million, the Indian at about 71 million, the Southern at about 22 million, and the Arctic at about 16 million square kilometres. These figures overlap at the Southern Ocean's boundaries, so they add up to more than the 361 million total — the Southern Ocean is defined by a current, not by coastlines, and it shares its water with the other three.
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| The five oceans by surface area. The Pacific alone covers more of the earth than all the land combined. Areas: NOAA / standard oceanographic values; the five figures overlap at the Southern Ocean's boundaries. |
The average depth of the ocean is 3,682 metres. Its deepest point is the Challenger Deep, in the Mariana Trench in the western Pacific, at 10,935 metres below the surface — deeper than Mount Everest is tall. Fewer than thirty people have ever been there. The ocean is, in other words, not a thin skin of water on a solid planet. It is a deep, three-dimensional world, and the part human beings live with — the surface, the coasts, the fisheries — is only its top edge.
2. Where the water came from
The earth was not born with oceans. When the young planet formed, some 4.6 billion years ago, it was too hot for liquid water to survive at the surface. The water arrived, and accumulated, by two routes that scientists still debate in their proportions. Some of it came from within: as the early earth cooled, water vapour trapped in the rocks was released by volcanoes — a process called outgassing — and condensed as the surface cooled enough for rain to fall and stay. Some of it may have arrived from outside, delivered by icy asteroids and comets striking the young planet.
Either way, once the rain began, it did not stop for a very long time. The first oceans formed perhaps 4.4 billion years ago, within a few hundred million years of the planet itself. And the water has been cycling ever since: evaporating from the surface, falling as rain, running back to the sea in rivers, and evaporating again. The water in your glass today has, in all likelihood, passed through an ocean, a cloud, a river, and the body of some living thing many times over. The ocean is not only the storehouse of the planet's water; it is the engine of the water cycle itself.
3. The zones: from sunlight to total darkness
Sunlight does not travel far in water. It fades with depth, absorbed and scattered, until below about 200 metres there is too little left for photosynthesis — the process by which plants and algae make food from light. Below about 1,000 metres there is no sunlight at all. On that single fact, oceanographers divide the open ocean into five depth zones.
The sunlight zone, from the surface to 200 metres, is where almost all ocean life lives — the plankton, the fish, the whales, the coral reefs. The twilight zone, from 200 to 1,000 metres, holds animals that have adapted to dim blue light: lanternfish with glowing organs, squid with enormous eyes. The midnight zone, from 1,000 to 4,000 metres, is permanently dark; its animals make their own light, through bioluminescence, and many of them have never been seen alive. The abyssal zone, from 4,000 to 6,000 metres, is near freezing and nearly empty of food, and the hadal zone — named after Hades — is the world of the deep trenches below 6,000 metres, where the pressure exceeds a thousand atmospheres.
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| The ocean in layers, from the sunlight zone to the hadal trenches. Band heights are schematic, not to scale. Zone boundaries and depths: NOAA Ocean Service. |
That darkness is not an absence of life; it is a different kind of life. The deep sea is the largest habitat on earth by volume, and scientists estimate that most of its species have not yet been described. Every expedition to a trench or a deep ridge returns with animals new to science. The ocean's darkness is, in a real sense, the largest unknown on the planet — larger than the unknown of the far side of the Moon, which at least can be photographed from orbit.
4. Why the sea is salty
Ocean water is, on average, about 35 parts of salt per thousand parts of water — about 35 grams of salt in every kilogram. Most of that salt is sodium chloride, ordinary table salt, but the sea also carries magnesium, calcium, potassium, and traces of almost every element known.
The salt came from the land. Rainwater is slightly acidic, because it absorbs carbon dioxide from the air. As it falls on rocks and flows through soil, it dissolves minerals and carries them to the sea in rivers. The rivers of the world deliver hundreds of millions of tonnes of dissolved salts to the ocean every year. The water then evaporates from the sea, leaving the salt behind — evaporation takes the water and leaves the minerals. Over hundreds of millions of years, the ocean accumulated its salt, and it has stayed roughly steady at today's level because salt is also removed: buried in sediments, trapped in sea-floor rocks, and carried back into the earth's interior where one tectonic plate dives beneath another.
Salinity is not the same everywhere. Where evaporation is strong and rain is rare — the subtropical Atlantic, the Red Sea, the Mediterranean — the water is saltier. Where great rivers pour in fresh water, or rain is heavy — near the mouth of the Amazon, in the polar seas where ice melts — it is fresher. And salinity matters for more than taste: saltier water is denser, so it sinks. That simple fact drives the deep circulation of the whole ocean, as the next sections show.
5. Waves, tides and currents
Three different motions stir the sea, and they work on three different scales.
Waves are mostly made by wind. Wind blowing across the surface pushes the water into ripples, and the ripples grow into waves. A wave is not water travelling across the ocean; it is energy travelling through the water, while each parcel of water mostly moves in a small circle and returns near where it started. A wave's height depends on the wind's speed, how long it blows, and the distance of open water it has to work with. Storm waves can reach 20 metres or more in the open ocean.
Tides are made by the Moon, with a smaller contribution from the Sun. The Moon's gravity pulls the ocean toward it, raising a bulge of water on the side of the earth facing the Moon, and a second bulge on the opposite side, where the earth is pulled away from the water. As the earth rotates beneath these bulges, most coasts see two high tides and two low tides each lunar day — a day measured not by the Sun, but by the Moon, lasting 24 hours and 50 minutes. When the Sun, Moon and earth line up, at new moon and full moon, their pulls add together and the tides are at their highest — spring tides. When they are at right angles, the tides are at their lowest — neap tides.
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| Animation: sea level over one lunar day (24 hours 50 minutes). Most coasts see two high tides and two low tides in that time, driven chiefly by the Moon's gravity. |
Currents are the slow rivers of the sea. Surface currents are driven by the wind and by the earth's rotation, and they move in great gyres: clockwise in the northern hemisphere, counter-clockwise in the southern. The Gulf Stream carries warm water from the Gulf of Mexico toward Europe at about 30 million cubic metres per second — more than twenty times the flow of all the world's rivers combined — and it is the reason western Europe is far warmer than other places at the same latitude. Below the surface, a slower circulation runs on density: cold, salty water sinks near the poles, creeps along the ocean floor, and rises again elsewhere, in a single great loop — the thermohaline circulation, sometimes called the global conveyor — that takes about a thousand years to complete one circuit. The ocean is not a still pond. It is a machine in constant motion, moving heat, salt, oxygen and nutrients around the planet.
6. The tsunami: when the sea rises as a wall
There is one more motion of the sea, and it is the one that shows what water can do when it is no longer gentle. A tsunami is not a tide and not an ordinary wave. A wind wave moves only the top skin of the sea; the water beneath it barely stirs. A tsunami moves the whole column of water, from the surface to the sea floor, because it is born not by wind but by the sea floor itself moving — in an undersea earthquake, a landslide, or a volcanic eruption, when a slab of the earth's crust suddenly lifts or drops and shoves the ocean above it aside.
In the deep ocean a tsunami is almost invisible. It races along at 700 to 800 kilometres an hour, as fast as a jet airliner, but its height is often less than a metre, spread over a wavelength of hundreds of kilometres, so a ship at sea would barely notice it passing underneath. The danger comes at the shore. As the water shallows, the wave slows and piles up, and the whole volume of the moving ocean comes ashore at once, not as a breaking wave but as a rising flood that keeps coming, wave after wave, for hours.
The scale of that power is hard to hold in the mind. Water is about eight hundred times denser than air: a cubic metre of it weighs a tonne. A tsunami 30 metres high arriving at the speed of a fast train carries a momentum that no wall, no building and no forest can stand against. On 26 December 2004, a magnitude 9.1 earthquake off Sumatra — one of the largest ever recorded — sent a tsunami across the whole Indian Ocean. Waves 30 metres high struck the coasts of Indonesia, Sri Lanka, India and Thailand, and the water reached as far as East Africa. About 230,000 people were killed in fourteen countries. The energy of the earthquake itself was, by a USGS estimate, equivalent to about 23,000 Hiroshima-type atomic bombs. Seven years later, on 11 March 2011, a magnitude 9.0 earthquake off Japan sent waves up to 40 metres high onto the coast of Tohoku, killing about 18,500 people and crippling the Fukushima nuclear plant. The ocean that feeds billions can, in a morning, erase a city.
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| The 2004 Indian Ocean tsunami: deaths by country. About 230,000 people were killed in fourteen countries. Figures: USGS / NOAA. |
The Qur'an knows this side of the sea. It is the same water that is subjected to us for food and trade, and it is also the water that drowned Pharaoh and his army when the sea parted and closed again — "and We parted the sea for you and saved you and drowned the people of Pharaoh while you were looking on" (Qur'an 2:50), the sea split "and each part was like a great mountain" (Qur'an 26:63). It is the water of the Flood, when "We opened the gates of the heaven with rain pouring down, and caused the earth to burst with springs, and the waters met for a matter already predestined" (Qur'an 54:11–12), and when the water overflowed, "We carried you in the sailing ship" (Qur'an 69:11–12). And it is the water of the storm at sea, where the Qur'an describes travellers in ships, sailing with a good wind, until "there comes a storm wind and the waves come upon them from everywhere and they assume that they are surrounded" (Qur'an 10:22). Waves from everywhere: that is, precisely, what a tsunami is — not one wave but a sea that has risen as one wall, coming from every direction at once, and no human power can hold it back.
The tsunami, then, is the other face of the ocean's might. The same laws that give us tides and currents, the same water that gives us half our oxygen, can be gathered by a single earthquake into a force that flattens coasts. The sea is a gift, and it is a power. The Qur'an holds both together, and the tsunami is where the two meet.
7. Pressure: the weight of the sea
Water is heavy, and the deeper you go, the more of it is above you. At the surface, the pressure is one atmosphere — the weight of the air. Descend 10 metres, and the pressure doubles. Every 10 metres adds about another atmosphere. At 200 metres, the bottom of the sunlight zone, the pressure is 21 atmospheres. At 1,000 metres it is 101. At the Challenger Deep, 10,935 metres down, it is about 1,094 atmospheres — more than a thousand times the pressure at the surface, or about a tonne pressing on every square centimetre.
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| Pressure in the ocean: about one more atmosphere for every 10 metres of depth. At the Challenger Deep it exceeds 1,000 atmospheres. |
No air-filled structure can survive that. A human diver with scuba gear can reach a few hundred metres at most, and only in a rigid submersible can anyone visit the trenches. Yet animals live there — fish, crustaceans, worms — with bodies that have no air spaces to crush, their tissues at the same pressure as the water around them. The pressure that would destroy a submarine is, to them, simply home. The deep sea is a reminder that the conditions of life are not fixed by what human beings can endure; life has found a way into every extreme the planet offers.
8. When two seas meet
Where two bodies of water with different salt and temperature meet, they do not mix at once. The denser water sinks below the lighter one, and a sharp boundary forms between them — a layer oceanographers call a halocline where the change is in salt, or a thermocline where it is in temperature. The waters meet, exchange slowly across the boundary, and keep their identities for a long way.
The classic example is the Strait of Gibraltar, where the Atlantic meets the Mediterranean. Atlantic water, cooler and less salty at about 36 parts per thousand, flows in at the surface; Mediterranean water, warmer and saltier at about 38 parts per thousand, flows out underneath, sinking below because its extra salt makes it heavier. The two flows pass each other in the same narrow strait, in opposite directions, separated by a measurable interface. The same pattern repeats wherever a river meets the sea: fresh river water, being lighter, spreads over the denser seawater, and a sharp halocline forms in the estuary. Fishermen have known these boundaries for centuries — the water changes colour, temperature and taste across a line you can sometimes see with the naked eye — and modern instruments can map them precisely.
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| When two seas meet: Atlantic water flows in at the surface of the Strait of Gibraltar while saltier Mediterranean water flows out beneath it, separated by a halocline. Schematic, not to scale; salinity values are typical for the strait. |
This is not a wall, and it is not permanent. Given time and turbulence, the waters do mix. The boundary holds for a while, across a distance, and then dissolves. That is exactly what makes it interesting: it is a meeting that is also a separation, a barrier that is real and measurable, and yet not absolute. Keep that image in mind. It matters for the second half of this post.
9. Life, food and oxygen
The ocean feeds more people than any other ecosystem. About three billion people get at least a fifth of their animal protein from fish and other seafood, and for many coastal communities the sea is the main source of animal protein. The catch is concentrated where the food is: where deep, nutrient-rich water rises to the surface — the upwelling zones off Peru, Namibia, California and the Arabian Sea — the plankton bloom, the fish gather, and the fisheries flourish.
But the ocean's gift to the air is even greater than its gift to the table. The tiny drifting algae of the surface — the phytoplankton — perform about half of all the photosynthesis on earth, and so produce about half of the oxygen in the air we breathe. Every second breath you take, on average, comes from the sea. The ocean also stores carbon: the same plankton carry carbon to the deep when they die and sink, and the cold deep water holds dissolved carbon for centuries. Without the ocean's appetite for carbon dioxide, the atmosphere would hold far more of it than it does.
That is why the state of the ocean is the state of the climate. The ocean has absorbed about a quarter of the carbon dioxide human beings have put into the air since the industrial revolution, and more than 90 percent of the extra heat trapped by greenhouse gases. It has done this at a cost: the surface water is warming, it is becoming more acidic as it absorbs carbon dioxide, and sea level is rising — about 3.6 millimetres a year in recent years, roughly 10 centimetres since satellite measurements began in 1993 — as the water warms and expands and as glaciers and ice sheets melt. The ocean gives, and it keeps the accounts.
10. The Qur'an and the sea
The Qur'an speaks of the sea again and again, and it speaks of it in three different registers: as a blessing, as a sign, and as a setting for human stories.
As a blessing, the sea is described as subjected — made serviceable — to human beings: "And it is He who subjected the sea for you to eat from it tender meat and to extract from it ornaments which you wear. And you see the ships ploughing through it, and [He subjected it] that you may seek of His bounty, and perhaps you will be grateful." (Qur'an 16:14) The same point is made again: "It is Allah who subjected to you the sea so that ships may sail upon it by His command and that you may seek of His bounty, and perhaps you will be grateful." (Qur'an 45:12) The food, the pearls, the ships and the trade are all named together, in one breath — the sea as provision.
As a setting, the sea carries some of the Qur'an's most vivid journeys: the ships "like mountains" that sail by Allah's leave — "And among His signs are the ships in the sea, like mountains. If He willed, He could still the wind, and they would remain motionless on its surface." (Qur'an 42:32–33) — and the meeting of the two seas where Musa, peace be upon him, travelled to meet his teacher: "And [mention] when Moses said to his boy, 'I will not cease [travelling] until I reach the junction of the two seas.'" (Qur'an 18:60) The phrase "junction of the two seas" — majma' al-bahrayn — is a place, a destination, and a mystery that the story never fully explains. The sea is where the known world ends and something else begins.
And as a sign, the sea is presented as evidence of order and of power held in check: the water sent down "in due measure" (Qur'an 23:18), the ships that sail "with what benefits people" among the signs for "a people who reason" (Qur'an 2:164). The Qur'an does not treat the sea as a backdrop. It treats it as a text to be read.
11. Darkness upon darkness
One verse describes the deep sea with a precision that has startled readers for centuries. Speaking of the deeds of those who deny the truth, the Qur'an says they are:
أَوْ كَظُلُمَاتٍ فِي بَحْرٍ لُّجِّيٍّ يَغْشَاهُ مَوْجٌ مِّن فَوْقِهِ مَوْجٌ مِّن فَوْقِهِ سَحَابٌ ۚ ظُلُمَاتٌ بَعْضُهَا فَوْقَ بَعْضٍ إِذَا أَخْرَجَ يَدَهُ لَمْ يَكَدْ يَرَاهَا
"Or [they are] like darknesses within an unfathomable sea which is covered by waves, upon which are waves, over which are clouds — darknesses, some of them upon others. When one puts out his hand, he can hardly see it." (Qur'an 24:40)
Consider what is described, layer by layer. First, the sea itself is deep — lujji, unfathomable, from a root meaning depth and darkness. Then there are waves above waves: not one wave, but a wave covered by another wave. Then clouds above that. Then darkness upon darkness — not one darkness, but darknesses layered on darknesses — until a man cannot see his own hand.
Modern oceanography knows each of these layers. Sunlight fades with depth and is gone below about 1,000 metres: the midnight zone is, literally, a darkness upon darkness, because the darkness is not one uniform black but a sequence of zones, each darker than the one above. The waves upon waves are also real, and in more than one sense: surface waves ride above internal waves — great slow waves that travel along the boundaries between water layers of different density, deep below the surface, some of them tens of metres tall and kilometres long, invisible from above but measurable by instruments. And the clouds above complete the picture: the light must pass through cloud, then through wave, then through deep water, each layer taking its share, until nothing is left.
No human being in the seventh century had been to the deep sea. The deepest a diver of that age could go was a few tens of metres, on a single held breath, in clear shallow water. The darkness of the deep ocean, its layered waves, and the fact that a man could not see his own hand there, were not things anyone could have observed from a boat. The verse describes a place its first listeners could not visit, in terms that match what instruments would measure fourteen centuries later. This is a correspondence, not a proof: the Qur'an is not a textbook of oceanography, and the verse's purpose is a moral one — it is a parable of spiritual darkness, not a scientific report. But the parable is built from a physical description that turns out to be exact, and that exactness is worth pausing on.
12. The two seas and the barrier
The other famous passage about the sea is the meeting of the two seas:
مَرَجَ الْبَحْرَيْنِ يَلْتَقِيَانِ بَيْنَهُمَا بَرْزَخٌ لَّا يَبْغِيَانِ
"He released the two seas, meeting [side by side]; between them is a barrier, so neither of them transgresses." (Qur'an 55:19–20)
The same idea returns twice more, with the fresh and the salt made explicit: "And it is He who has released the two seas, one fresh and sweet and one salty and bitter, and He placed between them a barrier and a prohibiting partition." (Qur'an 25:53) And: "And not alike are the two bodies of water. One is fresh and sweet, palatable for drinking, and one is salty and bitter. And from each you eat tender meat and extract ornaments which you wear." (Qur'an 35:12) A fourth verse adds the same barrier to the list of the signs of the One who made the earth a resting place: "and placed between the two seas a barrier?" (Qur'an 27:61)
What is the barrier? The text does not say a wall. The word is barzakh — a barrier, an isthmus, a partition — and hijran mahjuran, a prohibited partition. Read against the science of section 8, the description fits what oceanographers actually find: where fresh and salt water meet, or where two seas of different salt and temperature meet, a real boundary forms — the halocline — across which the waters exchange only slowly, keeping their identities for a long way. The fresh water of a river does not instantly become the sea, and the sea does not instantly become the river. At Gibraltar, the Atlantic and the Mediterranean pass each other in opposite directions for kilometres, each keeping its own salt and temperature.
But the honest framing matters here as much as in the earlier posts of this series. The barrier is not absolute, and the verse does not claim a wall of glass between the seas. The waters do mix, in time — that is also a measured fact, and the halocline is a gradient, not a seal. Some classical commentators read the barzakh differently, as the land between the seas, or as a divine restraint rather than a physical layer. The claim this post makes is a modest one: the Qur'an describes two seas meeting with a barrier between them that neither transgresses, and modern oceanography finds, at precisely such meetings, a real, measurable boundary that holds the two waters apart for a long way. That is a correspondence between an ancient text and a measured fact. It is not a derivation of oceanography from scripture, and it does not need to be. The point is that the description is recognisable to the science, not that the science was hidden in the verse.
13. The argument, and what the sea asks of us
Five points, then, in the shape of the argument this series has been building.
First, the sea is a system of exact measure. Its salt, its pressure, its tides and its currents all follow laws that can be written down and checked: pressure rising one atmosphere every ten metres, the Moon's pull giving two tides a lunar day, the Gulf Stream carrying a hundred rivers' worth of water toward Europe. The Qur'an's own word for this order is precise calculation — the same phrase used of the sun and the moon in the first post's series — and the ocean obeys it as faithfully as the heavens do.
Second, the sea is a place of hidden depths that the text describes truly. The darkness upon darkness of the deep sea, the waves upon waves, the barrier between two meeting seas — each of these is a description of a reality that no observer of the seventh century could have reached, and each matches what instruments measure now. The descriptions are not vague poetry that could fit anything; they are specific, layered, and checkable.
Third, the sea is presented as a gift with a condition. It is subjected to us — for food, for trade, for travel — and the verses that say so end the same way every time: "perhaps you will be grateful." Gratitude, in the Qur'an, is not a feeling. It is a practice: using a gift well, and not ruining it. The ocean that feeds three billion people and makes half our oxygen is now warming, acidifying, and rising, largely because of what we have put into the air. To receive the sea as a gift and then to damage it is not gratitude. The science of section 9 and the theology of section 10 point in the same direction here: stewardship is the proper response to subjection.
Fourth, the sea is a reminder of scale. Human beings live on the thin edge of a world that is mostly water, mostly dark, and mostly unknown. The ocean covers most of the planet, and we have explored a fraction of it. The Qur'an's sea passages do not flatter human knowledge; they place human beings small, in a boat, on a surface, above depths they cannot see into. Science says the same thing in its own language: the largest habitat on earth is the least explored. Humility before the sea is not a religious extra. It is the correct scientific attitude, too.
Fifth, the sea is a manifestation of might. The tsunami shows what the same water can do when it is gathered into a single force: a morning's wave that kills 230,000 people, a sea that rises as one wall and no human power can hold back. The Qur'an presents the same might in the Flood, in the drowning of Pharaoh, and in the storm where the waves come from everywhere. The ocean's power is not a separate subject from its provision; it is the same water, and the same Lord. To call the oceans mighty is not poetry. It is measurement.
The ocean, then, is not only the setting of the story. It is one of its arguments: a world of measure and darkness and meeting waters, described truly in an ancient text, and still, in every sense, deeper than we are.
Sources and notes
- Ocean coverage, volume, average depth (3,682 m) and deepest point (Challenger Deep, 10,935 m): NOAA Ocean Service, "Ocean Facts", accessed 7 November 2026. Everest height 8,849 m is the standard figure.
- Ocean areas by basin: NOAA / standard oceanographic values (Pacific 165.25, Atlantic 106.46, Indian 70.56, Southern 21.96, Arctic 15.56 million km²). The five figures overlap at the Southern Ocean's boundaries, so they sum to more than the ~361 million km² total ocean area.
- Origin of ocean water (outgassing and delivery) and the water cycle: standard account in NOAA and NASA Earth science materials; the relative contributions of outgassing and extraterrestrial delivery remain a subject of research, and the post says so.
- Depth zones and light penetration: NOAA Ocean Service, "Ocean Zones". Photosynthesis limit (~200 m) and absence of sunlight below ~1,000 m are standard values.
- Seawater salinity (~35 parts per thousand) and its sources: NOAA Ocean Service.
- Tides (two per lunar day of 24 hours 50 minutes; spring and neap tides) and the Gulf Stream transport (~30 million cubic metres per second): NOAA Ocean Service / National Ocean Service.
- Thermohaline circulation timescale (~1,000 years for a full circuit): standard oceanographic estimate; the figure is approximate and varies by pathway.
- Pressure with depth (~1 atmosphere per 10 m; ~1,094 atmospheres at the Challenger Deep): computed from the hydrostatic relation, pressure = 1 atm + depth/10 m.
- Strait of Gibraltar exchange flow and salinity contrast (Atlantic ~36, Mediterranean ~38 parts per thousand): standard oceanographic literature; values are typical, not single measurements.
- Phytoplankton producing about half of the world's oxygen; oceans absorbing about a quarter of human carbon dioxide emissions and more than 90 percent of excess heat; sea-level rise about 3.6 mm per year in recent years, roughly 10 cm since 1993: IPCC Sixth Assessment Report and NASA Sea Level Change / NOAA figures, accessed 7 November 2026.
- Internal waves travelling along density boundaries in the deep ocean: standard physical oceanography; they are distinct from surface waves and are measured by moored instruments and satellite altimetry.
- 2004 Indian Ocean tsunami (26 December 2004, magnitude 9.1 Sumatra-Andaman earthquake, about 230,000 deaths in 14 countries, waves to about 30 m; deaths by country: Indonesia 167,540, Sri Lanka 35,322, India 16,269, Thailand 8,212) and 2011 Tohoku tsunami (11 March 2011, magnitude 9.0, waves to about 40 m, about 18,500 dead and missing): USGS and NOAA / National Geophysical Data Center figures, accessed 8 November 2026. Tsunami speed in deep water (700–800 km/h) and wave behaviour: NOAA / National Ocean Service. Earthquake energy equivalent (about 23,000 Hiroshima-type atomic bombs for the 2004 earthquake): USGS estimate.
- Qur'an translations are adapted from Sahih International. The Arabic of 24:40, 55:19–20, 25:53, 35:12, 27:61, 16:14, 45:12, 42:32–33, 18:60, 2:164, 23:18, 10:22, 54:11–12, 69:11–12, 2:50 and 26:63 is quoted in full or in part as marked.
- The readings of 24:40 (deep-sea darkness and layered waves) and 55:19–20 / 25:53 (the barrier between two seas) are presented as correspondences between the Qur'anic text and measured oceanography, not as scientific proofs derived from the verses, and not as claims that the verses predict modern instruments. Classical commentators have read barzakh in more than one way, and the post notes that.
