Over half of Earth's surface is dark, flat seafloor few people have seen
Abyssal plains, lying three to six kilometres down, cover more than half the planet's surface and rank among the flattest places on Earth. Yet scientists only recognised them as distinct features in the late 1940s. Crushing pressure, about 750 times that at sea level, and total darkness keep them largely unexplored.
These plains form where new crust is born. Magma rises at mid-ocean ridges and cools into basalt, which seafloor spreading drags steadily sideways. That fresh crust starts out rough, but over time fine clay and silt bury the bumps. Much of the sediment arrives in turbidity currents, underwater avalanches funnelled from continental margins down submarine canyons; the rest is pelagic, material that drifts down from the open water above. Lumpy nodules rich in manganese, iron, nickel, cobalt and copper litter some areas.
They sit in the ocean's abyssal zone, between 3,000 and 6,000 metres. Above lies the bathyal zone, where temperature drops from 12 to 4 degrees Celsius, and below is the hadal zone of trenches, reaching about 11,034 metres at the bottom of the Mariana Trench. Sunlight fades long before: even in the clearest water, enough light for photosynthesis rarely reaches beyond 150 to 200 metres, and in murky coastal seas it may stop within a few tens of metres. With an average ocean depth of about 4,300 metres, the sunlit layer is a thin skin over a vast dark interior.
Life on the plains depends almost entirely on food sinking from that sunlit skin, so the abundance and makeup of the falling material shapes whole ecosystems. Changes at the surface, from climate shifts to fishing and ocean fertilisation, therefore echo down to the floor. Oxygen is scarce, much of it delivered long ago by cold polar waters, which makes the plains hostile to creatures from richer waters above. Even so, their sheer size makes them likely reservoirs of great biodiversity, and deep-sea corals grow at 3,000 metres and below.
The plains also help regulate the planet, influencing ocean carbon cycling, the dissolving of calcium carbonate and atmospheric carbon dioxide over centuries to a millennium. They leave a poor geological record, because subduction eventually swallows old ocean floor, and none had been studied systematically until recently.
Source: Abyssal plain
