For example, the water traveling along a tunnel or pipeline to a turbine in a hydroelectric generating station may be slowed suddenly if a valve in the path is closed too quickly. If there is 14 km (8.7 mi) of tunnel of 7.7 m (25 ft) diameter full of water travelling at 3.75 m/s (8.4 mph), that represents approximately 8,000 megajoules (2,200 kWh) of kinetic energy. This energy can be dissipated by a vertical surge shaft into which the water flows which is open at the top. As the water rises up the shaft its kinetic energy is converted into potential energy, avoiding sudden high pressure. At some hydroelectric power stations, such as the Saxon Falls Hydro Power Plant In Michigan, what looks like a water tower is in fact a surge drum.