The United States Navy is the most powerful blue-water navy in the world, and it's loaded with impressive warships of all kinds. The Navy's submarine fleet is substantial, consisting entirely of nuclear-powered submarines of various classes, from attack subs to nuclear launch platforms. One of the most impressive classes of fast attack submarines is the Seawolf-class, which was first introduced to the inventory in 1997 with the commissioning of the USS Seawolf (SSN-21).