Ocean Zones by Depth: The Five Layers of the Ocean, Explained

Scientists divide the open ocean into five layers by depth: the sunlight zone (epipelagic, 0 to 200 m), the twilight zone (mesopelagic, 200 to 1,000 m), the midnight zone (bathypelagic, 1,000 to 4,000 m), the abyssal zone (abyssopelagic, 4,000 to 6,000 m) and the hadal zone (hadopelagic, below 6,000 m, found almost only in trenches). The ocean averages about 3,682 m deep, and its deepest known point, the Challenger Deep, sits about 10,935 m down.

If you play Krillion, you already carry a depth gauge for all of this. Every point you score sinks your krill 10 metres, so 20 points takes you out of the sunlight zone, 100 points puts you in the midnight zone, and 380 points lands you beside the Titanic. Here is each layer with its real conditions, what lives there, and the score that gets you down to it.

• Sunlight zone (epipelagic): 0 to 200 m, or 0 to 20 points.
• Twilight zone (mesopelagic): 200 to 1,000 m, or 20 to 100 points.
• Midnight zone (bathypelagic): 1,000 to 4,000 m, or 100 to 400 points.
• Abyssal zone (abyssopelagic): 4,000 to 6,000 m, or 400 to 600 points.
• Hadal zone (hadopelagic): 6,000 m to the floor of the deepest trench, or 600 to about 1,094 points.

The lines are conventions rather than walls in the water. The top two are drawn by light: 200 m is roughly where too little sunlight remains for photosynthesis, and 1,000 m is roughly where the last trace of it is gone. The bottom two are drawn by the seafloor: 4,000 to 6,000 m is the depth of the great abyssal plains, and anything deeper is a trench.

One more number makes the game and the sea line up neatly. Pressure rises by about one atmosphere for every 10 m of seawater, so each Krillion point adds roughly one atmosphere of pressure on your krill. A 100 point dive ends under about 100 times the pressure at the surface.

This thin top layer holds almost all of the ocean's photosynthesis. Phytoplankton drifting here produce roughly half of the oxygen made on Earth, and nearly everything else in the sea eats, directly or indirectly, what they make. It is home to kelp forests, coral reefs, sea turtles, tuna, dolphins and the real Antarctic krill, which swarm in such numbers that they are among the most abundant animals on the planet.

Temperature here depends on latitude and season, from near freezing at the poles to around 30 °C in the tropics. Below a well mixed surface layer, many seas have a thermocline, a band where the water cools quickly with depth. By 200 m the pressure is about 21 atmospheres.

In the game you leave this zone fast. One Schooler (16 points) and one Plankton (4) makes exactly 20, so two ordinary answers take you out of daylight. A single Rare (32) on your first prompt drops you to 320 m.

The twilight zone is a blue dusk that fades to black. There is not enough light to grow anything, but there is enough to see a silhouette from below, so many animals here carry light organs on their bellies that glow to match the faint light above and erase their shadow. Lanternfish, hatchetfish and the vampire squid live here.

Every night the twilight zone empties upward. Huge numbers of fish, shrimp and squid rise toward the surface in the dark to feed, then sink back before dawn. This daily commute, called diel vertical migration, is often described as the largest migration on Earth by the mass of animals that make it. The water cools through this zone to around 4 to 5 °C near its floor, where the pressure reaches about 100 atmospheres.

To clear the twilight zone in the game you need 100 points. Seven Schoolers make 112, so a dive of steady middle answers does it. A single One in a Krillion (100 points) does it in one answer.

Below 1,000 m no sunlight arrives at all. The only light is made by animals: lures, flashes and glowing clouds. This is anglerfish territory, along with gulper eels and the giant squid that sperm whales dive down to hunt. The water is cold and steady, close to 4 °C, and food is scarce.

The deepest dive recorded for any mammal ends here. A 2014 study in PLOS ONE by Gregory Schorr and colleagues tagged Cuvier's beaked whales off California and logged one dive to 2,992 m, about 299 points in the game. Deeper still, the hydrothermal vents found in 1977 on the Galápagos Rift, about 2,500 m down, showed that whole communities of tube worms, clams and crabs can live on chemical energy instead of sunlight.

Two landmarks sit near the bottom of this zone. The average depth of the whole ocean is about 3,682 m according to NOAA, or roughly 368 points. The Titanic rests on the floor of the North Atlantic at about 3,800 m, so it takes 380 points to reach it: three One in a Krillions, one Rare and three Schoolers add up to exactly that. Fittingly, in our analysis of Krillion's prompt banks, "Titanic" is the answer that scores Plankton, the bottom tier, on more prompts than any other, 22 of them.

The abyss is mostly flat. Abyssal plains of fine sediment stretch across much of the deep ocean floor, built up over millions of years from marine snow: a steady fall of dead plankton, droppings and dust from the water above. That snow is almost the only food, and the animals here are built to wait for it. Sea cucumbers walk the mud and tripod fish stand on three long fins facing the current.

The water sits at around 2 to 3 °C and the pressure runs from about 400 to 600 atmospheres. In the game, 400 points means four One in a Krillions in a seven prompt dive. Reaching 600 means six.

The hadal zone takes its name from Hades, the Greek underworld. It exists almost only in trenches, the long gashes where one tectonic plate slides beneath another, and it covers a tiny fraction of the seafloor while holding nearly half of the ocean's depth range.

Life goes all the way down. In 2019 the Five Deeps Expedition filmed a dumbo octopus at 6,957 m in the Java Trench, the deepest octopus ever seen, as Alan Jamieson and Michael Vecchione reported in Marine Biology in 2020. That is about 696 points, four short of a perfect dive. In 2022 a team led by Jamieson filmed a juvenile snailfish at 8,336 m in the Izu-Ogasawara Trench off Japan, the deepest fish ever recorded, at about 834 points. Below that, small crustaceans called amphipods scavenge right to the floor of the Challenger Deep.

The bottom of the Mariana Trench's Challenger Deep was measured at 10,935 m, give or take 6 m, by Greenaway and colleagues in 2021, from pressure readings on crewed submersible dives. The pressure there is more than 1,000 times that at the surface. Jacques Piccard and Don Walsh first reached it in the bathyscaphe Trieste in January 1960.

A perfect classic dive in the Krillion game is seven One in a Krillions: 700 points, or 7,000 m. That lands you just inside the hadal zone, a few metres below the dumbo octopus. The Challenger Deep, at about 1,094 points, is out of reach in seven prompts. Only a long Endless dive gets there.

Here are the real landmarks as scores.

• Leaving the sunlight zone: 200 m, 20 points.
• Entering the midnight zone: 1,000 m, 100 points.
• Deepest mammal dive (Cuvier's beaked whale): 2,992 m, about 299 points.
• Average ocean depth: about 3,682 m, about 368 points.
• The Titanic: about 3,800 m, 380 points.
• Entering the abyssal zone: 4,000 m, 400 points.
• Entering the hadal zone: 6,000 m, 600 points.
• Deepest octopus filmed: 6,957 m, about 696 points.
• A perfect classic dive: 7,000 m, 700 points.
• Deepest fish filmed: 8,336 m, about 834 points.
• Challenger Deep: about 10,935 m, about 1,094 points.

The game's ocean has 138 hand-named zones, and the first 11,000 m or so follow the real sea: sunlit shallows, kelp, the twilight zone where daylight runs out, anglerfish territory, the Titanic, hydrothermal vents, the hadal zone and the Challenger Deep. Past that, Endless keeps going into places no submarine has seen, including a glass orchard, a drowned skyline, a coral parliament and a great whalefall, down to a shrine at 250 km, or 25,000 points.

The easiest way to feel how deep the twilight zone really is: play a dive and watch the gauge. Most players drift through it on ordinary answers, and every rare one you find is a real jump, 1,000 m for a One in a Krillion. Krillion Unlimited is free, needs no sign-up and has no daily cap, so you can chase the Titanic as many times as you like, or take Endless toward the trenches on three tanks of air. If you want help finding those deeper answers, our guide to getting better at Krillion covers the scoring ladder in detail.

NOAA, How deep is the ocean?

Greenaway, S. et al., on the depth of the Challenger Deep (Deep-Sea Research Part I, 2021).

Schorr, G. S. et al., First Long-Term Behavioral Records from Cuvier's Beaked Whales Reveal Record-Breaking Dives (PLOS ONE, 2014).

Jamieson, A. J. and Vecchione, M., First in situ observation of Cephalopoda at hadal depths (Marine Biology, 2020).