The depths of Earth’s oceans remain, to a large extent, as mysterious as the reaches of outer space. Cold, dark and hostile, less than 10 percent of their vast area has been explored by technology, and an even smaller fraction by human beings themselves.
Much of what we know about the sea, then, we know from deduction. Before the 21st century, for example, we had no photographic evidence of giant squid, nor even an intact carcass. But mariners from as far back as antiquity surmised their existence by the marks their tentacles left on the flanks of whales.
