In 1955, a thunderstorm in Belgium set off 40,000 pounds of buried explosives left over from WW1

21 mines were laid in preparation for the start of the Messines battle on June 7th 1917. However, only 19 of the mines were exploded. The rest remained hibernating underground for years after the end of the war.

In 1955, a freak thunderstorm assisted in detonating another mine. While surrounding damage was extensive, the only casualty was a cow. The resultant crater has since been filled in, although after heavy rainfall a slight depression in the ground indicates the location of the crater. The last mine remains undetected to this day.

(Source)