The mine casemate housed electrical generators, batteries, control panels, and troops that were used to test the readiness of the mines and to fire them when needed. The map of Boston Harbor's mine fields (below right) shows the harbor mine defenses consisting of 30 groups of mines, with 19 mines per group. Each mine was normally loaded with of TNT. So in Boston's case, a total of 57 tons of explosives guarded the harbor. As was the case with other parts of the U.S. coast defense system (e.g., its thousands of guns), there is no evidence that a mine was ever fired in anger against an attacking enemy, except in the Philippines in 1941-42. There is a report that a submarine attempted to penetrate the mine fields of Boston Harbor in June 1942, but reportedly withdrew before the mines could be fired or reset to detonate on contact.