The twilight zone is a part of the ocean 660 to 3,300 feet below the surface, where little sunlight can reach. It is deep and dark and cold, and the pressures there are enormous. Despite these challenging conditions, the twilight zone teems with life that helps support the ocean's food web and is intertwined with Earth's climate. Some countries are gearing up to exploit twilight zone fisheries, with unknown impacts for marine ecosystems and global climate. Scientists and engineers at Woods Hole Oceanographic Institution are poised to explore and investigate this hidden frontier.