Los Alamos laboratory main gate, 1943. One of the two security posts which guarded the only entrances to the Los Alamos laboratory in New Mexico, USA, during the Manhattan Project. The Los Alamos site, where the world's first nuclear weapons were designed and built in 1942-45, during World War 2, was reached by a rough, unpaved road. Each of the two security posts were manned by three soldiers. This underplaying of security probably helped to keep secret the true purpose of the site. However, the main technical area, well out of sight from casual observation, was ringed by a tall fence.