During WW I the British were taking a beating in the trenches of Belgium. There was a standoff and neither the British nor the Germans were making any encroachments on the other.
The British came up with the idea of making tunnels under the German lines and hollowing out chambers that were filled with an explosive much more powerful than TNT. The chambers (around ten) were then exploded and thousands of Germans were killed at one time. However not all of the chambers exploded and the tunnels collapsed rendering it impossible to reach the isolated chambers. One exploded long after WW II creating a large hollow that eventually filled with water. They know where the unexploded chambers are from maps recovered from the war office but they do not know if the explosives are still active or not. It is a game of wait and see just hoping they will not explode as several villages have been built up in the area. The original explosion was the largest man made explosion outside of an atomic bomb.
The two largest accidental explosions were during WW I in Halifax, Nova Scotia and in WW II at Pittsburgh,California both of which involved ammunition ships.
