These mines were fired electrically through a vast network of underwater electrical cables at each protected harbor. Mines could be set to explode on contact or be triggered by the operator, based on reports of the position of enemy ships. The networks of cables terminated on shore in massive concrete bunkers called mine casemates (see photo, below right), that were usually buried beneath protective coverings of earth. 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. Each protected harbor also maintained a small fleet of mine planters and tenders that were used to plant the mines in precise patterns, haul them back up periodically to check their condition (or to remove them back to the shore for maintenance), and then plant them again. Each of these harbors also had on-shore facilities to store the mines (called "torpedo storehouses") and the TNT used to fill them, rail systems to load and transport the mines (which often weighed over each when loaded), and to test and repair the electrical cables. Fire control structures were also built that were used first to observe the mine-planting process and fix location of each mine and second to track attacking ships, reporting when specific mines should be detonated (known as "observed fire"). The preferred method of using the mines was to set them to detonate a set period of time after they had been touched or tipped, avoiding the need for observers to spot each target ship. Each mine casemate controlled about 150 to 300 mines (depending on the harbor defended), arranged in groups of 19. The mines in a group were generally laid about apart, in lines running across the channel being protected, with or less on either end of the line. This meant that one group of mines could protect a total distance of about .