The hill 60 in the Ypres area was subject to a lot of mining, much of it done by Australians. Hundreds of tons of explosives were planted under German emplacements. Much of it (but not all of it) was detonated. When farmers returned to their land after the war, they rebuilt. Some of them didn't know they were rebuilding 90 feet above 20 thousand pounds of unexploded materials. One cache went off in a 1950s thunderstorm, the theory being that plowing had exposed the wires to a lightning strike.
Our guide for the day was Ian, a retired soldier from Northern Ireland who has written books on WWI. He led us past old German bunkers, showed us bomb craters 85 feet across, and answered (quite patiently) my thousand questions.