Most of us know more about other planets than the depths of the ocean. Despite covering nearly three quarters of the Earth, only around 5 per cent of the global ocean floor has been mapped in detail. And yet, with volcanoes, deep valleys, mountain peaks and vast plains, the topography of the ocean floor is as varied and magnificent as it is on land -- or on the surface of some celestial bodies. Jennifer Leake explores the challenges involved in mapping the underwater world and how developing a better understanding about what lies beneath the waves could help teach us about planet Earth.