Mysterious and seemingly monstrous beasts stalk the gloomy depths of the Atlantic Ocean, the so-called midnight zone where virtually no sunlight reaches. Temperatures are near freezing and the space between one creature and another can be vast.Scientists know very little about the species that inhabit this uninviting world more than a mile below the ocean's surface. For years it was believed that many of these fish nomadically wandered the ocean, munching on the organic debris that sinks from shallower waters. Now, a massive marine expedition has uncovered the secret sex lives of these fish, revealing that they may gather at underwater mountains to spawn. And the ecosystem turns out to be far richer than thought.The ongoing expedition has also collected 270 poorly known species and discovered 30 more. Deep-sea fish that spend the bulk of their lives swimming in open water are known as pelagic fish. Few large-scale expeditions have been dedicated to studying these creatures, and most species knowledge comes from what turns up in trawling nets.In fact, scientists know so little about these fish that 50 percent of the animals collected from deeper than 3,000 meters (1.86 miles) turn out to be unidentified species. Little is known about how they reproduce. Assuming the fish are few and far between down there, how do they rendezvous to allow a species to thrive?Using remotely operated vehicles, submersibles, massive trawling nets, and acoustic survey instruments, the Mid-Atlantic Ridge Ecosystems (MAR-ECO) team discovered that pelagic fish turn up in droves around seamounts and deep-sea mountain ranges, such as the Mid-Atlantic Ridge, to spawn.