Dredging, removal of silt, mud, and sand from harbors and navigation channels to keep them open for shipping. Of the 3 kinds of dredging vessels, the bucket dredger is the most widely used: A continuous moving chain of buckets extends down beneath the keel into the mud to be dredged. The grab dredger is a floating crane that drops a heavy scoop, or grab, into the mud and then hauls it to the surface. A more modern type is the suction, or hydraulic dredger which sucks up the mud through a pipe that is lowered to the seabed. The biggest hydraulic dredgers can dredge and discharge 20,000 tons or more in an hour. Dredgers may discharge their “spoil” into their own holds; into hopper barges moored alongside; or by means of a floating pipeline, directly onto the land. Dredgers are also used for mining deposits of heavy minerals that occur in alluvial gravels (deposited by running water). Tin, gold, platinum, and diamonds are often mined in this way.