Trenches and reefs

The seafloor has very deep, V-shaped areas called trenches, and the Mariana Trench (in the Pacific) is the deepest.

Its size is hard to imagine. The trench is over 2,000 metres deeper than Mount Everest is tall, and its deepest part, the Challenger Deep, reaches over 11,000 metres below sea level. Everest could sit inside it with 2 km of water above its peak.

Near-freezing temperatures, crushing pressure and complete darkness make it one of the harshest places on Earth.

And yet scientists have found unique life there. With no light, photosynthesis is impossible, so living things use energy from chemicals in the water to make food instead. A snail photographed on the Mariana seabed feeds on bacteria that do exactly this.

At the other extreme are reefs. A reef is a ridge at or near the ocean surface, made of rock or built by tiny sea animals called corals.

Reefs are among the most varied places for life on Earth. They are often called the 'rainforests of the sea', because they give food and shelter to about one-quarter of all ocean species.

The Great Barrier Reef, off Australia's northeastern coast, stretches over 2,000 km and covers about 350,000 square kilometres. It holds hundreds of coral species and thousands of fish species, and is a UNESCO World Heritage Site.

But reefs face serious threats from overfishing, pollution and climate change. When coral bleaches and dies, many sea animals lose their home, and the bleaching comes from changes in the chemistry of ocean water, largely caused by human actions.