Between 4,000 and 6,000 meters lies the abyssal zone, covering roughly 83 percent of the total ocean floor. The abyssal plains are vast, flat expanses of sediment that have accumulated over millions of years. Despite temperatures hovering near freezing and pressures that would crush most equipment, diverse communities exist here. Sea cucumbers, brittle stars, polychaete worms, and isopods patrol the seafloor. Some of these organisms have evolved gigantism — growing much larger than their shallow-water relatives.