Old Naval Mines Still Floating Around

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Figure 1: Old Training Mine Washes Up on North Carolina Beach. (Source)

I just read a news article about an old moored training mine washing up on a North Carolina beach (Figure 1). I am amazed at the number of mines that still wash up on beaches around the world. Figure 1 shows the mine that washed up on a North Carolina beach recently. Stories like this seem to happen frequently after severe storms. Fortunately, this training unit (i.e. inert) did not pose a hazard to the people who came upon it, but some explosive-laden mines still wash up on beaches (example).

Figure 2: Moored Mines As Portrayed in the Movie Destination Tokyo.  (Source)

This particular mine is similar to the type often portrayed in WW2 submarine movies (Figure 2). It looks like a floating metal sphere covered in "horns" that is moored to the ocean bottom. The horns are called Hertz horns (sometimes spelled Herz) and actually are a form of electric battery. When the horns are bent, such as when a ship collides with the mine, a vial containing sulfuric acid breaks, releasing the acid into a dry lead-acid battery (Figure 3). With the battery now filled with electrolyte, it begins generating a voltage which can be used to fire a detonator.

Figure 3: Schematic of a Hertz Horn. (Source)

This type of detonator can remain functional for decades. The Hertz horns were invented in 1868, which gives you an idea of how old naval mine technology is. It is estimated that 235,000 sea mines were laid during WW2 and some missed being cleared. While I consider this type of mine obsolete, they are still commonly used. Modern mines are much more capable, in some cases even being able to fire torpedoes (e.g US Navy Captor). They have onboard sonar systems that can track ships as they approach.

The mines washing up on beaches have been floating free in the ocean for various reasons. During WW2, minesweepers used paravanes (i.e. towed underwater gliders) to cut the mooring cables of the mines. With their mooring lines cut, the mines would float to the surface and a sailor would shoot a rifle at the mine. Their plan was for the bullets to either hit the Hertz horn and detonate the mine, or to put so many bullet holes in the mine that it would sink. Some floating mines were missed during this process and simply floated off to become a hazard later on. Other mines were missed and never had their mooring lines cut – eventually, the action of wind, waves, and corrosion would break the mine free. Also, these old mines have been laid as part of other conflicts, like the Korean War, and in the Persian Gulf to impede oil shipments. These mines have also ended up floating in the ocean.

Naval weapons are very powerful and even old ones pose a serious safety threat because explosives become unstable with age. One of these mines washed up on a beach and was mistaken for an old buoy– children were playing by it. It makes me shudder just thinking of it (Figure 4). Part of my concern for these kids is driven by a close call of my own involving an experimental torpedo. It is the only time in my life that I hoped my wife would be able to find my dental records.

Figure 4: Unidentified Sea Mine Near Children. (Source)