A dam was a structure built to control the flow of water.
In 1980, a military training site was built next to a dam in Caithness, Scotland. It was abandoned at some point before the dam was destroyed and thus flooded the area. (TV: Before the Flood) In 2119, an underwater mining facility known as the Drum stood in its place. The Twelfth Doctor and Clara Oswald visited the base and investigated a ghost invasion, caused by strange markings on the inside of an alien spaceship recovered from the water outside. (TV: Under the Lake)