Krillion Game

Ocean zones by depth

Updated

Scientists divide the open ocean into five layers by depth. Each one is darker, colder and under more pressure than the one above it, and each has its own kind of life. Krillion uses the same five layers as its depth zones: every point you score takes your diver 10 metres down, so a good dive is literally a trip through the ocean’s layers.

Here is what each zone is really like, and what it takes to reach it in the game.

The five ocean zones at a glance

  • Sunlight zone (epipelagic): 0–200 m. Enough light for photosynthesis. Krillion: from 0 points.
  • Twilight zone (mesopelagic): 200–1,000 m. Faint light that fades fast. Krillion: from 20 points.
  • Midnight zone (bathypelagic): 1,000–4,000 m. No sunlight at all. Krillion: from 100 points.
  • Abyssal zone (abyssopelagic): 4,000–6,000 m. No sunlight, near freezing. Krillion: from 400 points.
  • Hadal zone (hadopelagic): 6,000 m to about 11,000 m, only in trenches. Krillion: from 600 points.

The depth boundaries are the ones NOAA and the Woods Hole Oceanographic Institution use. The Krillion figures come from our game: depth is points × 10 m.

Sunlight zone (0–200 m)

The top 200 metres is where almost all the ocean’s visible light is. NOAA calls it the euphotic or sunlight zone: there is enough light for phytoplankton and seaweed to photosynthesise, and that makes it the base of most ocean food webs. It is also where the vast majority of commercial fisheries operate and where many protected marine mammals and sea turtles live.

Sunlight brings heat, so temperatures here vary widely with season and latitude. NOAA’s examples run from about 36°C in the Persian Gulf to about −2°C near the North Pole. Wind keeps the layer mixed. Below it sits the thermocline, where temperature drops quickly with depth.

In the game: every dive starts here. Two Surface answers (10 points each) are enough to sink you out of it.

Twilight zone (200–1,000 m)

Below 200 metres there is still a little light, but too little for photosynthesis. Organisms here depend on food drifting down from above. NOAA estimates that only about 20 percent of the surface’s primary production falls as far as this layer.

The twilight zone is far from empty. WHOI says it may hold more fish biomass than all the rest of the ocean combined. The bristlemouth, a small fish with a large jaw full of spiny teeth, may be the most abundant vertebrate on Earth. Bioluminescence starts to appear: many animals make their own light to hide, hunt or signal.

Every night, huge numbers of fish, squid and plankton swim up towards the surface to feed in the dark, then sink back before dawn. WHOI calls it the largest animal migration on the planet. Lanternfish, a family of more than 245 species, are a big part of it. In the 1950s, early sonar seemed to show the sea floor rising at night. It was a “false bottom” made by millions of migrating lanternfish.

Some animals here live in water with very little oxygen. Researchers at MBARI found vampire squid in Monterey Bay between about 600 and 900 metres, inside the oxygen minimum zone, living on a very slow metabolism.

In the game: 20 points gets you here. A dive of seven Sunlit answers (210 points) ends at 2,100 m, already past this zone.

Midnight zone (1,000–4,000 m)

Below 1,000 metres no sunlight arrives at all. NOAA calls this the aphotic zone: the only light comes from the animals themselves. The temperature stays close to 4°C, and at 4,000 m the pressure is over 5,850 pounds per square inch. Even so, sperm whales dive this deep to hunt.

The wreck of the Titanic lies in this zone, about 3,800 metres down.

In the game: 100 points, or one Abyssal answer, takes you to 1,000 m. To reach the Titanic’s depth you need 380 points.

Abyssal zone (4,000–6,000 m)

The abyssal zone is pitch black and close to freezing. The name comes from a Greek word meaning “no bottom”, because people once thought the ocean was bottomless. NOAA notes that three-quarters of the area of the deep-ocean floor lies at these depths: the vast, flat abyssal plains.

Food is scarce. About five percent of the surface’s primary production reaches the sea floor. Animals depend on “marine snow”, the slow fall of dead plankton and other particles.

In the game: 400 points. Seven Twilight answers (60 each) add up to 420 points and 4,200 m, just inside the abyss.

Hadal zone (6,000 m and deeper)

The deepest layer exists only in ocean trenches. NOAA’s figure for the very bottom, the Challenger Deep in the Mariana Trench, is 10,994 metres. The pressure there is over 8 tons per square inch.

Life still finds a way. Single-celled foraminifera have been found in the Challenger Deep. In 2022, a team led by the University of Western Australia filmed a juvenile snailfish at 8,336 metres in the Izu-Ogasawara Trench. Guinness World Records lists it as the deepest fish ever observed. This is why the deepest rank on our results screen is “a snailfish of the trenches”.

In the game: 600 points. Seven Midnight Zone answers (85 each) make 595 points, which stops you at 5,950 m, just 50 m short. Reaching the trenches takes at least six Abyssal answers.

Pressure: one atmosphere every 10 metres

Water pressure rises steadily with depth. NOAA puts it at one extra atmosphere (about 14.7 pounds per square inch) for every 10 metres. At 100 m you feel 11 atmospheres: one from the air and ten from the water.

That makes Krillion’s scale easy to picture. Every point you score is 10 metres, so every point adds roughly one atmosphere of pressure on your diver. A perfect seven-prompt dive reaches 7,000 m, where the water pushes with about 700 atmospheres.

Tiers and zones are different things

Our five answer tiers borrow the zone names (Surface, Sunlit, Twilight, Midnight, Abyssal), but a tier rates one answer while a zone tells you where your whole dive has reached. A run of modest Sunlit answers can still carry you into the Midnight Zone. One Abyssal answer is worth 100 points, the same as the depth of the twilight zone’s floor. The scoring guide explains the maths.

Practise

Ocean knowledge pays off in the game too. Try Name a sea: everyone says Mediterranean or Caribbean, while the Banda Sea and the Ligurian Sea score far more. Then take a dive in Krillion Unlimited and watch the zones change as you sink.

Sources

Frequently asked questions

What are the five ocean zones in order?

From the surface down: the sunlight zone (epipelagic, 0–200 m), the twilight zone (mesopelagic, 200–1,000 m), the midnight zone (bathypelagic, 1,000–4,000 m), the abyssal zone (abyssopelagic, 4,000–6,000 m) and the hadal zone (hadopelagic, 6,000 m and deeper).

How deep is the twilight zone?

The twilight zone runs from 200 to 1,000 metres below the surface. A little sunlight still reaches it, but too little for photosynthesis.

Which ocean zone is the Titanic in?

The midnight zone. NOAA gives the wreck's depth as about 3,800 metres, inside the 1,000–4,000 m bathypelagic layer.

How many points do you need to reach the Midnight Zone in Krillion?

100 points. Every point is 10 metres, so 100 points takes your diver to 1,000 m, the top of the Midnight Zone. One Abyssal answer is enough on its own.

Can you reach the Hadal Zone in a Daily Dive?

Yes, but only with mostly rare answers. The Hadal Zone starts at 6,000 m, which takes 600 of the 700 points available in a seven-prompt dive.

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