There are five oceans on the surface of planet Earth, the largest being named the Pacific Ocean, and between them they contain 97 per cent of the world's water. An ocean is a body of saline (salty) water, and although they cover 72 per cent of the planet's surface, we still know very little about what lies at the bottom of the deep blue sea - in fact we know of only around 230,000 living species of an estimated two million! Oceans aren't just the preserve of Earth though, other exoplanets and moons, such as Enceladus, are thought to have huge bodies of salty water.
Rising sea levels is one of the most worrying consequences of global warming, threatening over 100 million people living in vulnerable coastal areas. But measuring the rate of the rise is fraught with difficulty.