A large building made of gray stone blocks with green doors sits under a cloudy sky.
Diagnostic testing of the Fat Man weapon took place at this site.


Quick Facts
Los Alamos, NM

This site is on Los Alamos National Laboratory property and cannot be accessed by members of the public. 

Project Y ultimately developed two types of atomic bombs: a gun-type design that used uranium, and an implosion-type design that used plutonium. The plutonium bomb, nicknamed Fat Man, had a complex design that required more testing than the uranium bomb. At V-Site, the first assembly work for Fat Man occurred in 1944.   

Originally constructed for experimental work on special assemblies, V-Site underwent alterations in 1945 to facilitate work with explosive charges. Before the Trinity Test, project workers assembled the high explosives sphere for the Gadget at V-Site. Following the success of the Trinity Test, components of Fat Man received diagnostic testing at V-Site, ensuring that weapon components could endure the vibrations and cold temperatures they would experience in flight.  

Continue Your Journey 


Behind the fence, near V-Site, is S-Site which served as a high-explosives storage magazine during the Manhattan Project. Learn more about the history of the Manhattan by visiting the Bradbury Science Museum! The museum’s interactive exhibits share stories from the project and provide a glimpse of other “behind the fence” historical sites. 

Manhattan Project National Historical Park

Last updated: March 15, 2022