To construct its artificial islands, China builds atop already existing, islands, rocks, and even coral reefs. Building an island that can support airstrips and other military installations requires a lot of sand, however. To gather it, China uses a fleet of dredgers, ships designed to pick up and move materials from the seafloor. These dredgers use large tubes with cutting attachments at the end to grind up material on the seafloor and suck it up. From there, the material is carried through pipes or hoses and dumped on top of reefs, rocks, and other existing formations.