Deep Ocean Facts: Zones, Vents and How Much We Have Explored

The deep ocean is everything below about 200 metres, where sunlight fades too much for plants and algae to grow, and it makes up the large majority of the ocean's volume. It is cold, dark and under enormous pressure, yet it holds whole ecosystems, some of which run on chemical energy instead of sunlight. How much of it we have "explored" depends on what you count, and that question has a more interesting answer than the usual one-line statistic.

Below are the facts worth knowing, stated as carefully as we can.

#How big and how deep is the ocean?

The ocean covers about 71% of Earth's surface. Its average depth is roughly 3,700 metres, which surprises people who picture the sea as mostly shallow water with a few deep holes. It is the other way round. Most of the seafloor is a deep, broad plain, and the shallow continental shelves are the exception.

The deepest known point is the Challenger Deep, in the Mariana Trench in the western Pacific, at roughly 10,900 to 11,000 metres. Surveys disagree by a few dozen metres. Put Mount Everest (about 8,850 metres) at the bottom and its summit would still sit around two kilometres below the surface.

The longest mountain range on the planet is underwater too. The mid-ocean ridge system winds through every ocean basin for something like 65,000 kilometres. New seafloor forms there as tectonic plates pull apart.

#What are the zones of the ocean?

Scientists split the open ocean into layers by depth. The boundaries are approximate and the popular names are nicknames, but the scheme is a useful mental map.

A good way to hold this in your head: the sunlight zone is a thin skin. If the ocean at average depth were a swimming pool two metres deep, the part where photosynthesis happens would be roughly the top ten centimetres.

#How strong is the pressure down there?

Pressure rises by about one atmosphere for every ten metres of seawater. That rule of thumb makes the maths easy.

Worked example. At the ocean's average depth of about 3,700 metres, add roughly 370 atmospheres to the one atmosphere of air above, and you get around 370 times the pressure you feel at sea level. At the bottom of the Challenger Deep the figure is over 1,000 atmospheres, which works out to more than seven tonnes pressing on every square inch.

Deep-sea animals cope because they are not built like us. They have no air-filled lungs to crush, and their bodies are mostly water, which barely compresses. Many also carry molecules that help their proteins keep working under pressure. Researchers sometimes send polystyrene cups down on submersibles as a demonstration; the air is squeezed out of the foam and the cups come back shrunken to a fraction of their size.

#Is it completely dark?

Below about 1,000 metres, yes, as far as sunlight goes. But it is not a dead darkness. A large share of animals living in the open water column can make their own light, a trick called bioluminescence. They use it to find mates, lure prey, confuse predators, or hide their silhouette against faint light from above (counter-illumination).

One more fact that changes how you picture the deep: every night, huge numbers of small fish, krill and other animals swim up from the twilight zone toward the surface to feed, then sink back down before dawn. This daily vertical migration is often called the largest migration on Earth by the amount of living matter involved.

Most life in the deep depends on food from above. Dead plankton, faecal pellets and other debris drift down as "marine snow". When a whale dies and sinks, its carcass can feed a specialised community for years or even decades.

#What are hydrothermal vents?

Hydrothermal vents are openings in the seafloor, usually near mid-ocean ridges, where seawater that has seeped into the crust comes back out after being heated by magma below. The water can exceed 350 °C and still stay liquid because the pressure is so high. As the hot, mineral-rich fluid hits cold seawater, metal sulfides drop out and build chimneys. The darkest of these are nicknamed black smokers.

Vents were first observed directly in 1977, near the Galápagos Islands, by scientists diving in the submersible Alvin. They expected to find hot water. They did not expect to find life, and plenty of it: giant tube worms, clams, mussels and crabs clustered around the openings.

The explanation is chemosynthesis. Microbes use chemical energy, mostly from hydrogen sulfide in the vent fluid, to turn carbon dioxide into food. Giant tube worms have no mouth or gut as adults; they host these bacteria inside their bodies and live off what the bacteria produce. It was a major discovery because it showed that a whole ecosystem can run without sunlight at its base, which is one reason vents come up whenever scientists discuss where else in the solar system life might exist.

#How much of the ocean have we really explored?

You have probably heard that "we know more about the surface of the Moon than the ocean floor", or that only 5% of the ocean has been explored. There is truth in both, but they blur three different things.

• Mapped at low resolution. The entire seafloor has been mapped from space, using satellites that measure tiny bumps in the sea surface caused by the gravity of underwater mountains and trenches. The resolution is coarse, on the order of kilometres, so small features vanish.
• Mapped in high detail. Ships using sonar (multibeam echo sounders) produce much sharper maps. The international Seabed 2030 project reports that roughly a quarter of the seafloor has been mapped this way, and the share grows each year.
• Seen directly. The fraction of the deep seafloor actually observed by people, cameras or robots is far smaller. Estimates are rough, but it is a tiny percentage.

So the fair version is: we know the shape of the whole seafloor in blurry outline, a minority of it in detail, and very little of it by sight. The Moon comparison holds for the detailed maps, not for the outline.

People have reached the very bottom only a handful of times. Jacques Piccard and Don Walsh reached the Challenger Deep in the bathyscaphe Trieste in 1960. Nobody returned until the filmmaker James Cameron dived solo in 2012, and since 2019 more crewed dives have followed.

#Common misconceptions worth dropping

• "The deep sea is lifeless." Life is sparser than near the surface, but scientists find it at every depth they have sampled, including the trenches.
• "Deep-sea fish would explode if brought up." Dramatic, but not how it works. Fish without gas-filled swim bladders do not explode; they can be damaged by the change in pressure and temperature, and their tissues may sag or deform.
• "Most of the ocean's species are known." Expeditions to the deep regularly turn up animals new to science. Nobody knows the real total, and estimates vary widely.

#A good way to keep learning

The deep ocean rewards small, regular doses more than a single binge, because each fact makes the next one easier to place. Start with the zone table above, then follow one thread: vents, trenches or bioluminescence.

If you want that thread to arrive by itself, the ocean is one of the subjects in the Topicly topic catalog, and you can make it narrower with a custom topic such as "Life around hydrothermal vents" or "How deep-sea animals survive pressure". Under each discovery, Keep pulling this thread suggests related concepts to read next. The articles are written by AI and can contain errors, so treat anything important as a starting point and check it against a trusted source. For something further from home, our space facts for beginners make a good companion read.

Try this today: without looking, sketch the five ocean zones on a scrap of paper with rough depths, then check it against the table. Topicly is coming soon to iPhone and Android, and the download page has the details.