Oceans cover more than 70% of the Earth’s surface, containing roughly 97% of our water – and are an endless source of fascination. We know some of the creatures that dwell in the deep, from plankton to blue whales. We’ve seen pictures of the enormous Great Barrier Reef and of vast icy landscapes with icebergs and glaciers. Yet more than 80% remains unexplored, and still further swathes are rarely seen. Here we virtually plunge into the most mysterious parts of our watery world.