To get a sense of how much deep-sea action goes on in the far north, consider the Denmark Strait Cataract, located between Greenland and Iceland. It’s the planet’s tallest, mightiest waterfall, 11,500 feet high and a hundred miles wide, pumping five million cubic meters of water per second—and it lies two thousand feet beneath the ocean’s surface. Also, the Arctic is an undersea obstacle course of volcanic terrain. Peaks, ridges, valleys, faults, cliffs, shelves, vents: if the northern depths were music, they would be a live album by Nine Inch Nails.