One of the most powerful conventional weapons on Earth, an Ohio-class ballistic missile submarine, paid a port call in South Korea this weekend as a signal to the North. The vessel in question was the USS Michigan, which can let loose more than a hundred cruise missiles to destroy enemy air defenses and pave the way for manned aircraft to fly further bombing missions.

The Michigan surfaced in Busan, South Korea, for what was described by the Pentagon as a " routine " visit. Given the escalating tensions between North Korea and Trump's administration, though, the trip felt anything but.

Commissioned in 1982, Michigan was one of 18 Ohio -class ballistic missile submarines that provided an important second-strike nuclear capability. They carried out "doomsday patrols" designed to ensure that, in the event of a surprise attack by the Soviet Union, enough nuclear weapons would survive at sea to launch a devastating counterattack.

The end of the Cold War in 1991 reduced the number of submarines needed to 14. Four of the older submarines, including the Michigan, were converted into cruise missile submarines by taking the hull space devoted to their Trident II D-5 nuclear ballistic missiles and converting it to house Tomahawk conventional land attack cruise missiles instead. The result is a single submarine that can carry 154 land attack cruise missiles, each of which can travel up to 900 miles and attack a target with a 1,000-pound high explosive warhead with GPS-level accuracy. It's an incredible package of precision firepower that rivals the destructive capability of entire 20th century air forces.

Tomahawk cruise missile being reloaded on the island of Guam. U.S. Navy photo by Mass Communication Specialist Seaman Apprentice Samuel Souvannason.

A submarine like Michigan would participate in a theoretial first wave of attacks against North Korea. Flying low and slow to avoid radar, its Tomahawk missiles can be expended on well-defended targets without risk of losing a pilot. Tomahawks would be used to strike known headquarters, air defense radars, missile sites, and airfields, suppressing Pyongyang's air defenses. That way the next attack wave, consisting of manned U.S. aircraft, would have an easier time of it. Although North Korea's air defenses are generally old and can avoided by flying at high altitudes, some systems, such as the KN-06 long-range surface to air missile could bring down U.S. aircraft.

Here's a video of Michigan's sister ship, USS Florida, launching two Tomahawks.

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Michigan's visit comes as the Trump administration is turning up the heat on North Korea, with Trump himself saying that the U.S. needs to be prepared "for the worst" . The administration also sent the aircraft carrier USS Carl Vinson to the region and has just activated a Terminal High Altitude Area Defense (THAAD) anti-ballistic missile system to protect South Korea .

Michigan has not escaped North Korea's notice. Pyongyang's state media reportedly warned "the USS Michigan won't even be able to rise to the surface when it will meet a miserable end and turn into an underwater ghost." North Korea's ability to detect and strike the Michigan, which could sit hundreds of miles off the Korean peninsula to attack targets on land, is essentially zero. North Korea has no long-range anti-submarine warfare force to speak of. The U.S. Navy won't have a "ghost submarine" anytime soon.

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