Commonly, naval mines were set up in harbor regions to act as deterrents, and to impede land invasion by sea. They were designed to sink ships or other seaborne craft, or as a threat to prevent the usage of an area of water. Moored mines were tethered to sinkers, and floated at predetermined depths. The firing mechanisms of mines were either traditional pressure points, which detonated the explosive on contact. However, some harbor mines were controlled by a pressure plate at its top, which could be turned off to allow transit of friendly ships.