A curious feature of our planet's surface is that it has two distinct levels: the dry land on the continents, on average a few hundred metres above sea level, and the ocean floor, making up two-thirds of the Earth's surface, several kilometres below sea level. Only in the past fifty years have scientists begun to explore in detail this vast region, revealing beneath the waves a landscape quite unlike the world we are used to. They have discovered a vast mountain range which encircles the entire globe. Here new sea floor is being continuously formed as the Earth's surface splits apart.