Oceans: the blue planet

The ocean covers about 71% of Earth's surface (NOAA).
The five oceans are the Pacific (largest and deepest), Atlantic, Indian, Southern and Arctic (smallest).
The deepest known point is the Challenger Deep in the Mariana Trench in the western Pacific, nearly 11,000 m deep.
The seabed has continental shelves (shallow edges of continents), deep flat abyssal plains, mid-ocean ridges where plates move apart, and deep trenches where plates sink.

Climate: ocean currents move heat around the world. The Gulf Stream and North Atlantic Drift keep the UK mild. The ocean also absorbs a large share of the extra heat and carbon dioxide people add to the air.
Food and jobs: fishing, shipping, tourism and energy (such as offshore wind).
Coral reefs cover less than 1% of the ocean floor but support about 25% of all marine species (NOAA). The Great Barrier Reef off Queensland, Australia, is the world's largest coral reef system.

Overfishing: catching fish faster than they can breed. The UN Food and Agriculture Organization (FAO) reports that more than a third of assessed marine fish stocks are overfished.
Plastic pollution: plastics break into tiny microplastics that are eaten by sea life.
Warming and acidification: warmer water causes coral bleaching (corals push out the algae that feed them and turn white); extra carbon dioxide makes seawater more acidic.
Solutions: Marine Protected Areas, fishing quotas, cutting single-use plastics and reducing carbon emissions.
Countries control fishing and resources in an exclusive economic zone (EEZ) up to 200 nautical miles from their coast, under the UN Convention on the Law of the Sea.