Zones of the Ocean: A Journey From Sunlight to Darkness

If you dropped straight down from the ocean's surface near Puerto Rico, you'd fall through water for over five miles before hitting the bottom of the Puerto Rico Trench — deeper than Mount Everest is tall. Along the way, sunlight disappears, temperatures drop near freezing, and the pressure crushes anything not built to survive it. Yet every single layer is full of life.

- The four main depth zones of the ocean and what defines each one - Which animals live in each zone and how they've adapted - Why sunlight — not just water — controls where life can thrive - How scientists explore zones humans can't survive in unprotected

Scientists call this the epipelagic zone, but most people just call it the sunlight zone. This is where sunlight penetrates strongly enough for photosynthesis, so it's home to nearly all ocean plant life — including phytoplankton, tiny drifting organisms that produce about half of all the oxygen on Earth. This zone holds the ocean life most people picture: sea turtles, dolphins, tuna, and coral reefs, which only grow here because reef-building coral needs sunlight to survive.

Below 650 feet, sunlight fades to a dim blue glow and then to nothing — this is the mesopelagic, or twilight, zone. No plants can grow here because there isn't enough light for photosynthesis, so animals must eat each other or drift-down food falling from above, called 'marine snow.' Many twilight zone animals, like the lanternfish, produce their own light through a chemical reaction called bioluminescence — using it to attract prey, confuse predators, or find mates in near-total darkness.

Past 3,300 feet lies the bathypelagic, or midnight zone, where sunlight never reaches at all and water temperatures hover just above freezing. Animals here, like the anglerfish, have slow metabolisms, huge mouths, and often glow to lure prey in permanent blackness. Go even deeper — past 20,000 feet — and you reach the hadalpelagic zone, named after Hades, the Greek god of the underworld. This zone exists only in ocean trenches, like the Mariana Trench, where pressure is over 1,000 times what we feel at the surface. In 2012, filmmaker James Cameron piloted a submarine solo to the bottom of the Mariana Trench — nearly 36,000 feet down — and still found living creatures.