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In the lead-up to the cancelled invasion of the home islands of Japan in World War II, the U.S. Navy dumped close to 25,000 mines in the water as part of 1945’s Operation Starvation. Hundreds remained in the water long afterwards were still around decades later, still dangerous.
What makes sea mines so insidious is the mine designers cracked the overarching technical problem of keeping the mines effective after they’ve been in the ocean for years.
The most basic type of sea mine is detonated when a ship brushes up against the side and causes an electrical circuit to be completed that activates the explosive.
Saltwater is rough on just about everything and corrosion quickly made the first mechanical circuits ineffective after not very long.