During a scuba dive in the area known as the Shallow of Vada south of the city of Livorno I came across a naval mine from the Second World War that remained unexploded; over time it sank near a cliff at a depth of about 50 meters. From historical information I learned that a minefield had been built in the area to prevent the navigation of military and civilian ships in transit to the port of Livorno. The area after the war had been reclaimed but some mines were not removed and with time they sank due to the erosion of the shell due to sea water. Due to its strategic value, the area was particularly guarded also by the air forces of the warring factions and there are reports of a fierce bombardment of a marine convoy of transport ships escorted by Italian Navy naval ships. We have testimony from the many wrecks of sunken ships present in the stretch of sea that goes from San Vincenzo to Livorno and from the aero bombs still present in the seabed. Some ships in transit in the minefield are sunk by mines with serious consequences; as it is easy to imagine.
I have taken from Wikipedia the system that was used for anchoring naval mines to the seabed.
Moored contact mines with plummet
Sequence of laying a moored contact mine with a plummet
A special form of moored contact mines are these with a plummet. When the mine is launched (1), the mine with the anchor floats first and the plummet sinks into the depth (2). In doing so, the plummet unwinds a wire, the deep line, which is used to sets the depth of the mine below the water surface before it is launched (3). When the set length has been unwound, the anchor is flooded and the mine is released from the anchor (4). The anchor begins to sink and the mooring cable unwinds until the lead anchor reaches the sea floor (5). Due to the decreasing tension on the deep rope, the mooring cable is clamped. The anchor sinks further down to the bottom of the sea pulling the mine as deep below the water surface as the deep line has been unwound (6). Thus, even without knowing the exact depth, an exact depth of the mine below the water surface can be set. Limited only by the maximum length of the mooring cable.