High pressure, 'low lives'

Stand on a beach, and the air above you presses down with a force of one atmosphere. Dive into the sea, and that pressure doubles by the time you are just 10 metres down. It keeps adding up; every 10 metres brings another atmosphere. By 1,000 metres, the pressure is 100 atmospheres. The ocean averages about 3,700 metres in depth; so, much of its floor sits under around 370 atmospheres. In the deepest part of the ocean, the Challenger Deep in the Mariana Trench, it is 1,100 atmospheres. That is like trying to balance a car on your finger!

Yet, life is abundant at these depths. Bacteria thrive in the mud of the trenches, amphipods scour the floor of the Challenger Deep for food, and fish, squid and sea cucumbers make the abyssal plains their home. Some of these creatures spend their entire lives in this extreme environment, never venturing away. Others — certain fish and invertebrates — commute up and down the water column every day. And then there are mammals such as whales and seals, which must surface to breathe but dive deep, sometimes for hours, to find food.