the center of American nuclear weapons development, and the Theoretical Division was for many years the center of the laboratory. The Laboratory made great strides in improving the weapons, making them easier to manufacture, stockpile and handle. The Operation Sandstone tests in 1948 demonstrated that uranium-235 could be used in implosion-type nuclear weapons. Mark played a key role in the development of thermonuclear weapons in the early 1950s. A crash program to develop the hydrogen bomb was approved by President Harry S. Truman in January 1950 at Edward Teller's urging before the laboratory had a workable design. This put the laboratory under great pressure. Hans Bethe recalled that "There was a lot of controversy in Los Alamos in those days, and Carson always was imperturbable and stood in the middle of it and had a balanced judgment." When Stanislaw Ulam finally came up with a workable design, it was Mark that he approached first. Mark took the Ulam design to Bradbury, and they put the idea to Teller. "Teller then completed and extended the invention," Bethe noted, "so, in this case, Carson was the mediator between two people who really didn't like each other." "Within an hour of Carson's ... remarks," Teller recalled, "I knew how to move ahead". The Teller-Ulam design would become that of all thermonuclear weapons. When it came to testing the design in the Ivy Mike nuclear test, Mark again had a crucial go-between role. Bradbury placed Marshall Holloway in charge, but Holloway and Teller did not get along. Mark later recalled that: The Ivy Mike test was successful, obliterating an island in Enewetak Atoll in the Pacific on November 1, 1952. Frank Harlow, who joined the Theoretical Division in 1953, and was appointed as a group leader by Mark in 1959, noted that Mark took a personal interest in the computer systems, then a rapidly developing field. As most weapon research in the 1960s no longer involved the Theoretical Division, Mark branched out, sponsoring research into hydrodynamics, neutron physics and transport theory. He also supported Frederick Reines's research into neutrinos, for which Reines was awarded the Nobel Prize in Physics in 1995. In July and August 1958, and again the following year, Mark was a scientific adviser to the United States delegation at the Conference of Experts on Detection of Nuclear Explosions. Delegates from Western and Eastern bloc countries discussed detection methods in the context