More people have stood on the moon than journeyed to the lowest point of the ocean, which is called Challenger Deep, 11km below sea level. Find out more about the alien landscape at the bottom of the sea.
Around the coastline is the continental shelf, an area of shallow sea which surrounds most land on Earth. It stretches out about 60km from the coasts and drops down to around 200m deep.
This area of the sea is called the epipelagic zone, the sunlight zone. It is light enough for plants to grow, whereas all the deeper layers are too dark for this.
The epipelagic zone is home to more recorded species than any other part of the ocean, because it is better known than the other zones. However, many thousands of species may await discovery deeper in the ocean.
At the edge of the continental shelf, the sea bed gradually falls away into the deep ocean. This is called the continental slope.
The slopes vary enormously in steepness and in some places they are divided by dramatic canyons with nearly vertical walls.
As you go deeper, the sea gets darker. The upper regions of the continental slope, called the mesopelagic zone, the twilight zone, are still quite light during the day but the lower boundary, about 1km deep, is perpetually dark.
At a depth of around 3.5 to 4km the sea bed flattens out. This is the abyssal plain, the largest habitat on Earth. It covers over half the ocean floor and depths reach down to 6km.
In some small areas of the oceans, particularly in the western Pacific, the sea floor drops away even further into long trenches, with water depths of 10 to 11km.
The abyssal plain receives no sunlight at all. It is completely dark apart from occasional flashes of ghostly blue light produced by bioluminescent animals.