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.