Under the sea, all the way at the bottom, is what marine researchers refer to as the benthic zone. It’s the lowest layer of the ocean — of any body of water, really — including any sediment. It’s home to many bottom-dwelling critters, like sponges and sea worms, that experts call benthos. The region is not always as deep down as you’d think, either. In oceans, the benthic zone starts at the shoreline and follows the continental slope to the deepest depths.