Trinity Base Camp
In late 1944, soldiers, technicians, and scientists moved to Trinity Base Camp constructed by the Army at the former Dave McDonald ranch house 10 miles from ground zero. 
By early summer of 1945, it became a bustling hive of activity with more than 200 soldiers, scientists and technicians working at a feverish pace. 
The area in the vicinity of the McDonald ranch house was built up to support hundreds of personnel. Although Base Camp may have looked similar to a ramshackle Hooverville, constant traffic to and from the test site meant that this location was always buzzing with activity. 
Security was of paramount importance for the Trinity Test. There was always a possibility that enemy and even allied spies were paying close attention to the next generation bomb.
Base camp is where soldiers, technicians, and scientists worked and lived while preparing for the Trinity Test. Located 10 miles southwest of ground zero, it was constructed mostly of temporary prefabricated buildings taken from Civilian Conservation Corps camps by Army Corps of Engineers contractors. It had a mess hall, barracks, shower facilities, and laboratories.
The first to arrive were twelve Military Police (MPs) from the 1st MP Detachment, 4817th MP Unit from Los Alamos including then-Corporal Marvin Davis. The unit had been sent from Fort Riley, Kansas to Los Alamos in 1943. They were reassigned to Trinity in December 1944 to guard the site. Thirty more MPs arrived in January with First Lieutenant Howard Bush, a reserve officer from Brooklyn, assigned as the Chief of Security and Base Commander. There were about 50 MPs by the end of the test, including a stable crew of 3. Each of the men and Bush, now a Captain, received a one-grade promotion when assigned to Trinity.
MPs set up check points near ground zero and as far away as Mockingbird Gap. It took a War Department pass stamped “T” to get access to Trinity. Once at the checkpoint, the pass was exchanged for a Trinity badge that had to be worn at all times on the site. Different badges allowed access to different parts of the site.
The MPs named the roughly north-south road, which ran between base camp and ground zero, Broadway. The main east-west roads were named Vatican and Pennsylvania Avenues.
Security was an overriding concern. The MPs and soldiers in the Provisional Engineer Detachment did not know their destination until they arrived and did not have a real understanding of its purpose until the explosion, seven months later. Logistics, procurement, receiving, communications, personnel were all complicated by security. Efforts to conceal the test site meant that nothing could point to the site’s actual purpose.
There were émigré scientists at the site, some classified as enemy aliens. The soldiers said the scientists had a one-track mind and were completely focused on the test. Enrico Fermi was said to have the widest grasp of the project at the site. Emigrating from Italy in 1938, he built the first atomic pile at the University of Chicago in 1942. He was thought of as saintly and infallible, which led to his nickname “The Pope”.