Into the Dark

Photosynthesis stops entirely by two hundred metres down. Despite being a thin skin on a vast ocean, this layer holds an estimated 90% of all marine life. Surface temperatures near the equator reach 20–30°C before dropping sharply at the thermocline.

Sunlight fades to total darkness by a thousand metres; only a faint blue thread of it survives this far. Each night, trillions of animals rise from here toward the surface to feed, then sink back before dawn — the largest animal migration on Earth, by weight of life moved. Water hovers between 4–20°C.

Zero sunlight reaches here — every point of light from now on is made by something alive. Pressure climbs past 100 atmospheres, water settles near a constant 4°C, and most bioluminescence glows blue-green, the one wavelength that still travels this far. A rare few species instead glow red, invisible to nearly everything that might be watching.

This single zone covers more of Earth's surface than any other habitat — roughly 60% of the planet. Near-freezing at 2–3°C, under more than 600 atmospheres of pressure. Food is scarce; most life here survives on "marine snow," the slow drift of organic debris falling from the world above.

Named for Hades. Confined to narrow trenches where tectonic plates collide. The Challenger Deep, in the Mariana Trench, is the deepest known point on Earth at 10,935 m — Everest would fit inside with over two kilometres to spare. Pressure here exceeds 1,100 atmospheres: roughly the weight of fifty jumbo jets, pressing on a single person.