registered himself as available for scientific war work, but heard nothing for several months after the outbreak of the Second World War in September 1939. He was then approached by Geoffrey Taylor. Taylor was an expert on fluid dynamics, and was dealing with more questions from government departments regarding the effects of explosions than he had time to answer. He asked Penney if he could investigate the behaviour of an underwater explosion. Penney became a member of the Physics of Explosives Committee (Physex), and reported his results to another committee, Undex, which was run by the Admiralty and was interested in underwater explosions such as those created by mines, torpedoes and depth charges, and their effects on the hulls of ships and submarines. Most of the data on underwater explosions was from the First World War. With Royal Navy engineer officers Penney designed and supervised development of Bombardon breakwaters, steel structures that formed part of the Mulberry harbours that were placed off the Normandy beaches after the D-Day invasion. These mobile breakwaters protected landing craft and troops from the Atlantic rollers. Penney's job was to calculate the effects of waves on the Bombardons and devise the most efficient arrangement of them. Manhattan Project The August 1943 Quebec Agreement provided for British support of the American Manhattan Project, which aimed to develop atomic bombs. Over the objections of the Admiralty and Imperial College, Penney was sent to join the team of British scientists at the Manhattan Project's Los Alamos Laboratory in New Mexico, where expertise on explosions and their effects was in demand. At Los Alamos Penney gained recognition for his scientific talents, and also for his leadership qualities and ability to work in harmony with others. Within a few weeks of his arrival he was added to the core group of scientists making key decisions in the direction of the programme. Major General Leslie Groves, the director of the Manhattan Project, later wrote: One of Penney's assignments at Los Alamos was to predict the damage effects from the blast wave of an atomic bomb. Soon after he arrived at Los Alamos, he gave a talk on the subject. Fellow Manhattan Project scientist Rudolf Peierls recalled that: Penney's wife never recovered from post-natal depression after Christopher's birth, and died on 18 April 1945. He arranged for Joan Quennell, a nurse, to look after the boys. He wanted to return home, but