This list of military nuclear accidents is petrifying and doubly interesting for the number of warheads and kg's of Plutonium that were never recovered.
This is one good example but there are dozens more over on Wikipedia.
January 22, 1968 – 7 miles (11 km) south of Thule Air Force Base, Greenland, a fire breaks out in the navigator’s compartment of a B-52 which crashes, scattering three hydrogen bombs on land and dropping one into the sea. During a cleanup complicated by Greenland’s harsh weather, contaminated ice and aircraft debris are buried in the U.S. Bomb fragments were recycled by Pantex, in Amarillo, Texas. Danes were outraged by the event because Greenland is a Danish possession, and Denmark forbids nuclear weapons on its territory. Denmark had massive demonstrations against the U.S. One warhead was recovered by Navy SEALs and Seabees (U.S. naval engineers) in 1979. An August 2000 report suggests that the other bomb remains at the bottom of Baffin Bay.