Human beings have climbed the highest mountains, been to the poles and crossed jungles and deserts - but we have only just started to explore its seas. Relatively speaking, we have paddled round its edges and skimmed its surface. But huge parts of the ocean floor remain uncharted territory, and could yet yield some amazing discoveries.
Dr Robert Ballard made one of them. He may not be a household name but the ship he found in 1985 - Titanic - certainly is.
Dr Ballard, 57, has also led dives on the Bismarck and the Titanic's sister ship, the Britannic, in the Aegean Sea, during a career that has earned him the epithet the Indiana Jones of the deep. This summer he is part of an expedition to the Black Sea which is investigating the theory that the story of Noah's flood could be based on real events more than 7,000 years ago.
Sediment samples from the sea suggest that around 5500BC, waters from the Mediterranean surged through the narrow opening of the Bosphorus and into the Black Sea, then a much smaller freshwater lake, causing it to rise by 150 metres and flood large tracts of land. The expedition will attempt to prove the theory by locating drowned settlements. Until recently, archaeologists could work in only 200 feet of water, giving them access to only four per cent of the ocean bed. But they can now go down to depths of 20,000 feet - opening up more than 90 per cent of the world's seas.
Advances in submarine technology and sonar detection are opening up new frontiers in underwater exploration. Unfortunately, those same advances have made wrecks accessible to looters and profiteers, prompting campaigns for better protection for marine treasures.
Dr Ballard says virtual exploring - "telepresencing" as he calls it - by remote control cameras and submersibles will make the sea bed the discovery zone of the 21st century. Finding and examining ancient ships helps piece together the movements of the earliest civilisations. But exploration of the vast underwater mountain ranges and valleys can help our understanding of continental movements, earthquakes and volcanoes (there are many more active volcanoes in the sea than on land).
In 1977, Dr Ballard was part of the team that discovered geothermal vents, nutrient-rich currents of hot water that emerge from below the ocean floor and support previously unseen forms of sealife such as transparent crabs and 12-foot worms. The find backed up theories of plate tectonics and prompted speculation that these deep-sea colonies could have been the starting point of evolution.
"We know more about the surface of the moon or some of the distant planets than we do about the ocean floor," Dr Ballard says. "And I always say the ocean's bottom is more interesting than the moon's behind."