The five depth zones

Oceanographers split the water column into layers by how much light reaches them. The names vary a little between books, but the usual five are:

• Epipelagic (sunlit zone), 0 to about 200 m. Enough light for photosynthesis. Almost all plankton that make food from sunlight live here, and so do most fish, turtles and mammals you know.
• Mesopelagic (twilight zone), about 200 to 1,000 m. A faint blue glow fades away with depth. There is too little light for photosynthesis, but animals can still see silhouettes against the dim sky.
• Bathypelagic (midnight zone), about 1,000 to 4,000 m. No sunlight at all. The only light is made by animals.
• Abyssopelagic (the abyss), about 4,000 to 6,000 m. Cold, dark and under a pressure of several hundred atmospheres.
• Hadopelagic (the trenches), below about 6,000 m. Found only in deep trenches, mostly where one tectonic plate slides under another. The name comes from Hades.

The boundaries are not sharp. Light does not switch off at 200 m; it fades smoothly. Different textbooks use slightly different depths, and the real light limit depends on how clear the water is, so treat the numbers as rounded guides.

As you go down, two things change steadily: the temperature falls (the next lessons show how) and the pressure rises (by about one atmosphere every ten metres). Life gets thinner too, because almost all food is made in the sunlit layer. But thinner is not the same as empty. Life has been found in every zone, down to the deepest trench.