The serious exploration of life in the ocean is just getting under way. Nearly every inch of Earth’s lands have been mapped, and most have been crisscrossed many times by human observers. Meanwhile, only about 5 percent of the ocean below the surface waters has ever been seen at all, let alone fully explored. There are 1.25 million terrestrial species on Earth, but there may be as many as 50 million species in the sea, of which only about 250,000 have been discovered and named so far.