  Criticality experiments with $^{235}$U (metal and hydride) and $^{239}$Pu
(metal) were performed during the Manhattan Project. Results from these
experiments provided necessary information for the success of the Manhattan
Project. These experiments have been previously described in compilations made
after the Manhattan Project, but those works are either lacking in technical
details or are not publicly available. This work aims to provide detailed
information while showcasing the enduring impact of these experiments 75 years
after they were performed. Furthermore, we use modern computational methods
embodied in the MCNP6 code and ENDF data to analyze and interpret these
historic measurements. The world's first four criticality accidents are also
discussed, as lessons learned from these helped shape the field of criticality
experiments.
