The Earth’s seas and oceans, which cover 70% of its surface, are home to a large amount of life. With an average depth of 4 km, they account for 97% of the water on the planet. In addition, they feature some impressive landforms: the most extensive mountain range, the Mid-Oceanic Ridge, is deep under the sea. Four times as long as the Andes, the Rocky Mountains, and the Himalayas arranged in a row, it stretches 65,000 km across the Arctic and the Atlantic.