Everyone knows certain things can be easily compressed, that is, their volume can be made smaller. Gases like air are easily compressed. Solids, including liquids, are nearly incompressible. The scientists at Los Alamos chose to try to compress Plutonium in order to get it to reach "critical" mass, to create a sponraneous chain-reaction, a nuclear explosion. But, they knew it would take enormous pressure, perhaps millions of pounds of pressure per square inch to do it. If this image works, it is taken from Los Alamos records, and is a series of X-ray photographs taken a few millionths of a second apart, showing a hunk of metal being compressed by use of high-explosives detonated all around the object, a process they called "explosive lenses".
This compression was required to bring the 11KG mass of PU-239 to criticality, where there were more neutrons available for fission than those flying out of the mass with no collisions. This is the detonation system used in the device in the Trinity test at Alamogordo, N.M. on July 16, 1945, and in "Fat Man" at Nagasaki on Aug. 9, 1945. Sorry I couldn't be more specific..... Hal