Swimming Sea Cucumber © R. Mickens/AMNH

At depths below 4,000 meters (2.5 miles), the sea floor is called the abyssal plain. This distant realm was once thought to be lifeless. When life was found there, scientists still considered it a sparse environment with little diversity. But the abyssal plain turns out to be one of the most diverse ecosystems on Earth.

What accounts for this surprising diversity of life?

Astonishing Diversity
Slight differences in the distribution of sea-floor nutrients create distinct habitats as different from one another as forests and deserts.

Tiny particles of food - once assumed to blanket the abyssal plain evenly - can clump together and fall in thick blizzards, creating uneven patches of nutrients on the floor below. Additionally, ocean currents, mudslides and animals moving over the bottom can pile up or churn sediments, creating drifts, mounds and depressions where detritus settles.

The surprising variation in the abyssal plain habitat and intense competition for resources have produced an unexpected diversity of mud-dwelling animals; more than 200,000 species have already been found. Once thought to be completely devoid of life, the deep sea floor is now compared to such famously diverse ecosystems as coral reefs and tropical rain forests.