[myknowledge]

Probably a sea mine dropped in World War II or less likely in the Korean War, since there were no significant naval clashes at that time.

From the colonization of 1910 through to the Japanese surrender in 1945, Korea was ruled by Japan, and in a harshly brutal manner.

I'd hypothesize that the mine which sank the ROKS Cheonan was laid by the Japanese in WWII.

[Huebolt]

The cost of producing and laying a mine is usually anywhere from 0.5% to 10% of the cost of removing it, and it can take up to 200 times as long to clear a minefield as to lay it.

Parts of some World War II naval minefields still exist, because they are too extensive and expensive to clear; some of these mines might remain dangerous for hundreds of years (Wiki, unfortunately...)