On the night of February 5, 1958 a B-47 Stratojet bomber carrying a hydrogen bomb, off the Georgia coast collided with an F-86 Saberjet fighter at 36,000 feet. The collision destroyed the fighter and severely damaged a wing of the bomber, leaving one of its engines partially dislodged. The bomber’s pilot, was instructed to jettison the H-bomb before attempting a landing. The incident is the definition of a Broken Arrow scenario — a situation where a nuclear weapon is released, but without intent to harm. Richardson dropped the bomb into the shallow waters of Wassaw Slough, near the mouth of the Savannah River, a few miles from the city of Tybee Island, where he believed the bomb would be swiftly recovered. Recently declassified documents show that the bomb was an “Mk-15, Mod O” hydrogen bomb, weighing four tons, packing more than 100 times the explosive punch of the one that incinerated Hiroshima and capable of creating a 20-30 kilometer thermal blast radius.

Soon search and rescue teams were sent to the site. Wassaw Slough was mysteriously cordoned off by Air Force troops. For six weeks, the Air Force looked for the bomb without success. Underwater divers scoured the depths, troops tromped through nearby salt marshes, and a blimp hovered over the area attempting to spot a hole or crater in the beach or swamp. Then just a month later, the search was abruptly halted. The affair of the missing H-bomb was discreetly covered up. The end of the search was noted in a partially declassified memo from the Pentagon to the Atomic Energy Commission, in which the Air Force politely requested a new H-bomb to replace the one it had lost. The bomb still lies somewhere off the coast 57 years later, with a 2001 recovery effort carrried out by the United States Air Force unsuccessful. The current location of the MK-15 nuclear bomb is unknown, thanks to the passage of time and the twenty-three hurricanes and tropical storms that have hit the area since 1958. The condition of the bomb is an enigma — the outer metal alloy shell should be fine if it is resting in a coffin of silt. But if the bomb has been disturbed and came in contact with salt water, the metal would eventually erode, allowing the contents to seep out and distributing uranium into the water.

Reference:

http://www.mapsofworld.com/on-this-day/february-5-1852-the-hermitage-museum-opens-to-the-public-in-st.-petersburg-russia 

http://www.counterpunch.org/2013/09/25/the-case-of-the-missing-h-bomb-2/

http://io9.com/5906826/when-we-lost-an-unexploded-nuclear-bomb-off-the-coast-of-georgia