Most people live and work on land. Some journey by air, or go to sea. But all return to the land. Our terrestrial view of the world defines exploration.
As we spread and settle on land that was once wilderness, there seems little left on Earth to explore. We know that space is still a vast, mysterious frontier. But we seldom think of the oceans as watery terra incognita, blinkered as we are by our land view.
However, the sea floor is an extension of the land. It has mountains, valleys and plains, although they are hidden by seawater. Little is known about the deep recesses of either the sea or the sea bed.