he deepest-diving whales go no further. At the very bottom, more than 11km down, lie the Challenger Deeps. Twelve humans have walked on the Moon. None has set foot in the Deeps, and only two have seen them with their own eyes.
Yet things live down there. Big things. Microphones throughout the sea listen for enemy submarines, but no one has explained the undersea roar that occasionally startles listeners. The sound appears biological in origin, and its wavelength suggests that it is produced