Another venue, the energy research facility Project Pegasus, was also briefly used, though the unsuitability of such an institution for use as a general prison led to the imprisonment of most criminals there being discontinued eventually. The US Government then set about building a unique penitentiary dedicated and designed exclusively for the detainment of super-human criminals. Using expertise, research and technology pioneered at Project Pegasus spearheaded by Dr. Henri Sorel, and extremely robust materials such as adamantium and osmium steel, they built an underground three-level structure over below ground level in the Rocky Mountain range in Colorado.