Thousands of years ago, people looked out across an ocean and asked themselves, 'What is on the other side?' And the bravest of them began to travel and find the answers - beautiful islands, frozen lands, different peoples . . . And there are still interesting questions about the oceans. How do they change our weather? Why does the water go up and down twice a day? Why do most animals and plants live near the land? And what can possibly live at the bottom of the ocean, thousands of metres down, where there is no light? Surely nothing can stay alive in a place like that.