The use of moored mines in the Baltic Sea is known since 1848. A moored mine consists mainly of three components: the mine case, the anchor and the anchor rope. The mine case is made of a spherical or oval metal container and contains between 30 and 350 kg of explosives as well as the trigger and a security device. Due to its size, the mine case has enough buoyancy so that it can be mounted under water.
There is a variety of trigger mechanisms used in moored mines. The most common are pressure triggers, which detonate when the mine comes into contact with for example a ship. Therefore, the mine case is equipped with lead caps, which contain a glass container filled with an electrolyte. When a ship touches and bends one of those lead caps, the glass container breaks and the electrolyte is released into a dry cell generating an electric potential which then triggers the explosive charge. Another pressure sensitive trigger mechanism is using a pendulum to break the glass container with the electrolyte, which is now located inside the mine case. Horned moored mines are also quite common. Here, the mine case also contains a charged battery, while the horns are mounted onto switches that are flipped when a ship hits one of the horns to close an electric circuit and trigger the explosive charge.
With the beginning of the Second World War, moored mines were also equipped with acoustic, magnetic and electronic sensors to facilitate the detonation of the mines without a direct contact between the ship and the mine. Acoustic sensors were developed, which recognize ship noises and trigger the mine on a specific intensity of those noises. Mines equipped with magnetic sensors are triggered by the approach of an iron ship and thus the change in the vertical magnetic field strength.
The security device is built into moored mines to disarm the mine in case it is detached from the anchor, for example by sinking the mine through flooding. The anchor consists basically of a weight, which is sunk on the seafloor and also holds the steel rope on which the mine case is fixed at a certain, pre-defined water depth. This anchor rope is also a weak point of the moored mine, since it might rip due to the influence of currents or corrosion or cut during mechanical minesweeping and the mine gets relocated and is still dangerous in case the security device fails.
(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)