An acoustic mine is one which is actuated by the noise made by a ship passing through the water. This noise is a combination of the noise of the water drops falling off the hull and the sound of the propellers and machinery. The magnetic mine is activated by a change in the earth’s magnetic field due to the presence of a large metal conductor, such as a ship. Moored mines can be laid in deeper water, but there is a limit to the length of moorings that are feasible. The US have very few active minesweepers or hunters although is possibly due to the fact that the wider oceans of the Atlantic and the Pacific do not lend themselves to mine warfare in the same way as the shallow waters of the North sea, Channel and parts of the Mediterranean.