The conveyance of coal by means of water flowing in pipes. Coal may be pumped to the surface in shallow mines, but beyond 150 ft (46 m) or so of depth, there are technical difficulties. Solids handling pumps rarely deliver against heads exceeding about 200 ft 1557 (61 m). Two such pumps, placed in series, have been used in Trelewis Drift, Wales, to pump out slurry.