The explosion occurred below the water line and left a saucer-shaped crater on the seabed deep and across. Background The December 1938 discovery of nuclear fission by Otto Hahn and Fritz Strassmann—and its explanation and naming by Lise Meitner and Otto Frisch—raised the possibility that an extremely powerful atomic bomb could be created. During the Second World War, Frisch and Rudolf Peierls at the University of Birmingham calculated the critical mass of a metallic sphere of pure uranium-235, and found that instead of tonnes, as everyone had assumed, as little as would suffice, which would explode with the power of thousands of tonnes of dynamite. In response, Britain initiated an atomic bomb project, codenamed Tube Alloys. At the Quebec Conference in August 1943, the Prime Minister of the United Kingdom, Winston Churchill, and the President of the United States, Franklin Roosevelt, signed the Quebec Agreement, which merged Tube Alloys with the American Manhattan Project to create a combined British, American and Canadian project. The British contribution to the Manhattan Project included assistance in the development of gaseous diffusion technology at the SAM Laboratories in New York, and the electromagnetic separation process at the Berkeley Radiation Laboratory. John Cockcroft became the director of the joint British-Canadian Montreal Laboratory. A British mission to the Los Alamos Laboratory led by James Chadwick, and later Peierls, included scientists such as Geoffrey Taylor, James Tuck, Niels Bohr, William Penney, Frisch, Ernest Titterton, and Klaus Fuchs, who was later revealed to be a spy for the Soviet Union. As overall head of the British Mission, Chadwick forged a close and successful partnership with Brigadier General Leslie R. Groves, the director of the Manhattan Project, and ensured that British participation was complete and wholehearted. With the end of the war the Special Relationship between Britain and the United States "became very much less special". The British government had trusted that America would share nuclear technology, which the British saw as a joint discovery, but the terms of the Quebec Agreement remained secret. Senior members of the United States Congress were horrified when they discovered that it gave the British a veto over the use of nuclear weapons. On 9 November 1945, the new Prime Minister of the United Kingdom, Clement Attlee, and the Prime Minister of Canada, William Lyon Mackenzie King, went to Washington, DC, to confer with Truman about future cooperation in nuclear weapons and nuclear