Bikini Atoll, Marshall Islands, in the Pacific Ocean. This is the site where the US detonated a hydrogen bomb (nuclear bomb) on 1st March 1954. The unexpectedly large explosion and subsequent public outcry provoked diplomatic negotiations that ended in the 1963 Nuclear Test Ban Treaty prohibiting surface tests. The explosion, expected to produce 4 to 6 megatons of explosive force, instead produced 15 megatons. The mushroom cloud reached 40 kilometres into the air - well into the stratosphere - and eventually spread to four continents. It was the largest above-ground test of a hydrogen bomb in US history.