The fire that caused $400 million in damage to a US nuclear submarine last month may have been caused by no greater a menace than a vacuum cleaner. That is the preliminary result of a naval investigation of the May 24 fire, which occurred while the USS Miami was in a Maine shipyard for routine maintenance. The fire affected the sub's living quarters, command and control spaces, and torpedo room, reports CNN. The vacuum was used "to clean work sites at end of shift" and then stored "in an unoccupied space," according to a navy news release. Fortunately, the submarine's nuclear propulsion systems were off at the time of the fire and there were no torpedoes or other weapons aboard. (More submarine stories.)