Abyssal plain A flat region of ocean floor lying between 2000 and 6000 metres (6500–20,000 feet) deep. It is covered by a thick layer of mud called ooze. The animals that live here must lift themselves out of the ooze, burrow inside it, or find a way of slithering across it.

Abyssal zone The layer of water that extends from 4000 metres (13,000 feet) down to the ocean floor. Only a few animals live in this zone.

Bathypelagic zone The layer of water between 1000 and 4000 metres (3300–13,000 feet) deep. To blend in with their environment and so avoid predators, most creatures in this zone are dark brown, purple or grey in colour.

Benthic zone The ocean floor. It may be bare rock, coral or a thick layer of mud.

Black smoker A dark, rocky “chimney”, found along mid-oceanic ridges. Black smokers form above cracks, called vents, in the seabed through which hot, sulphur-rich water spurts out. A number of creatures flourish in the waters near the vent. They eat bacteria that feeds on the sulphur-rich waters, or other bacteria-eating animals. The most amazing of these creatures are 3-metre-long (10-feet) tube worms.

Continental shelf The part of a continental landmass that lies under ocean waters. The continental shelf has an average depth of 200 metres (650 feet) below the surface.

Continental slope The steep part of the continental shelf that plunges down towards the abyssal plain. A constant “rain” of decomposed material from the surface settles on the slope to form a soft, muddy ooze.

Deep-sea trench A very deep valley in the seabed, plunging to depths of 6000 to almost 11,000 metres (20,000–36,000 feet). Trenches are formed when the huge plates that make up the Earth’s crust push together, forcing one to slide beneath the other.

Epipelagic zone The sunlit top 200 metres (650 feet) of ocean water. In these waters, there is enough light for plants to make food via photosynthesis. The richest variety of life in the oceans occurs in this zone.

Hadal zone The water found in deep-sea trenches. The zone is named after Hades, the Greek god of the underworld. Incredibly, some animals have been found to live at these depths. They can survive the great pressure because their bodies have no air spaces inside them.

Littoral zone The seashore, the part of the sea closest to the land.

Mesopelagic zone The layer of ocean water that lies between 200 and 1000 metres (650–3300 feet) deep. It is also known as the twilight zone.

Mid-Oceanic Ridge A long mountain range running along the ocean floor. The ridge is formed where lava erupts between the edges of two plates that make up the Earth’s crust.

Pelagic zone The area of open water beyond the seashore. It includes the entire ocean except for the ocean floor.

Plankton Tiny plants, known as phytoplankton, and animals, known as zooplankton, that float or swim in the surface waters of the ocean.

Photosynthesis The process by which green plants use sunlight as an energy source to turn carbon dioxide and water into the sugars they need for food. Photosynthesis in the ocean can occur only in clear, shallow waters.

Phytoplankton Microscopic plants that float in the ocean currents. Phytoplankton use sunlight and nutrients dissolved in ocean water to make food by photosynthesis. They comprise most of the plant material found in the oceans.

Zooplankton The tiny animals that feed on phytoplankton. Zooplankton includes the larvae (young) of fish, as well as tiny relatives of crabs and shrimps called copepods. They provide food for a range of ocean animals.

Consultant: Chris Jarvis