The first operable remotely triggered mines were developed and used during the First World War. A ground mine is a weapon, which is deployed on the bottom of the sea, a river or inside a harbor area and is triggered without any contact between the mine and an approaching vessel. Ground mines can be deployed from ships, submarines and planes.
Ground mines typically have an elongated, cylindrical metal case, but also box- and semi-spherical-shaped casings were built and used, each equipped with an explosive charge between 65 and 880 kg and an either acoustic or magnetic trigger device.
Mines equipped with magnetic sensors are triggered by the approach of an iron ship and thus the change in the vertical magnetic field strength. Acoustic trigger devices use hydrophones to detect the noise of an approaching ship. In addition, a pressure sensor was developed to measure the changes in water pressure that are caused by a ship crossing over the mine. Usually, a combination of two or more trigger devices is used in ground mines to maximize their efficiency against ships. Some mines are also equipped with a counting mechanism that detonates only after a pre-defined number of ships crossed over the mine, which also complicates the delaboration process. Furthermore, some mines contain “counter measurements” against conventional delaboration, like for example opening the mine case.
All ground mines are equipped with a power supply (e.g. zinc-carbon battery), that provides the trigger mechanisms with energy. Once the battery is empty, the mine can no longer be detonated. However, some mines could additionally be triggered from land using an ignition cable. The walls of the mine case were often quite thin, which is why the contained explosive charges can occasionally be found lying on the seafloor today.
(see: Böttcher et al., (2011): Munitionsbelastung der deutschen Meeresgewässer – Bestandsaufnahme und Empfehlungen; Bund/Länder-Messprogramm für die Meeresumwelt von Nord- und Ostsee (BLMP) im Bundesamt für Seeschifffahrt und Hydrographie (BSH) (Hrsg.); Hamburg; www.munition-im-meer.de)