Naval mines still remain in that sea area.
The navy laid naval mines around the harbor.
The minesweeper cleared the naval mines from the waters.
The cargo ship struck a naval mine and sank.
In the middle of the night, we heard the sound of a naval mine exploding.
機雷 is a mine placed in water to sink ships, while 地雷 is a mine buried on land to injure people or vehicles.
Abbreviation of 機械水雷 (きかいすいらい, 'mechanical mine'). The kanji 機 (mechanism) and 雷 (thunder/explosive) combine to suggest a mechanical explosive device.