When the German Army closed in on Paris during the Second World War, Pontecorvo, his brother Gillo, cousin Emilio Sereni and Salvador Luria fled the city on bicycles. He eventually made his way to Tulsa, Oklahoma, where he applied his knowledge of nuclear physics to prospecting for oil and minerals. In 1943, he joined the British Tube Alloys team at the Montreal Laboratory in Canada. This became part of the Manhattan Project to develop the first atomic bombs. At Chalk River Laboratories, he worked on the design of the nuclear reactor ZEEP, the first reactor outside of the United States that went critical in 1945, followed by the NRX reactor in 1947. He also looked into cosmic rays, the decay of muons, and what would become his obsession, neutrinos. He moved to Britain in 1949, where he worked for the Atomic Energy Research Establishment at Harwell.