The open ocean is mainly barren, because cold, nutrient-rich currents are confined to deep water, far beneath the reach of plankton. Seamounts—which stand up to 13,000 ft (4,000 m) above the sea bed—form a major obstruction to these currents, diverting them and pushing them upward. This brings an upwelling of nutrients into the sunlit zone, and allows phytoplankton to flourish. As these nutrient-rich currents rush over the top of the seamount, they split into two and sweep around it. This makes the water above the seamount rotate, encircling a cylindrical column of still water that extends high above the height of the seamount. This “virtual” cylinder is called a Taylor Column. Above a seamount, it forms an area of back-eddies and still water in which nutrients accumulate and plankton get trapped. This creates a zone of incredible richness and productivity above the seamount—an “oasis” in the nutrient desert of the open ocean.