The Trump administration released a report on the state of America's nuclear weaponry on Friday. The assessment, known as a Nuclear Posture Review, mainly concerns U.S. nukes and missiles. But buried in the plan is a mention of a mysterious Russian weapon called "Status-6." On paper, at least, Status-6 appears to be a kind of doomsday device. The report refers to it as "a new intercontinental, nuclear-armed, nuclear-powered, undersea autonomous torpedo." "The radius of total or near-total destruction is the size of a pretty large metropolitan area, actually," says Edward Geist, a Russia specialist at the Rand Corp. who has...