A hydrogen bomb is missing from the United States' arsenal and has been, evidently, for 40 years. When last seen, the bomb was one of four aboard an Air Force B-52 bomber that crashed on a frozen bay near Thule Air Force Base in northern Greenland on Jan. 21, 1968. Two years later, the United States and Denmark reported that they agreed that the accident caused no danger to man or animal and plant life in the area." The 96-page report of the investigation indicated that all four nuclear warheads aboard the plane had disintegrated on impact. Case closed. Well, maybe not, the BBC says this week. Declassified documents that the BBC obtained under the United States Freedom of Information Act indicate that only three of the bombs were accounted for, and that the United States searched secretly for the fourth bomb, without success. By April [1968], a decision had been taken to send a Star III submarine to the base to look for the lost bomb, which had the serial number 78252. (A similar submarine search off the coast of Spain two years earlier had led to another weapon being recovered.) But the real purpose of this search was deliberately hidden from Danish officials. One document from July reads: "Fact that this operation includes search for object or missing weapon part is to be treated as confidential NOFORN", the last word meaning not to be disclosed to any foreign country. "For discussion with Danes, this operation should be referred to as a survey repeat survey of bottom under impact point," it continued. And what does the Pentagon have to say about all this now? It had no comment for the BBC.