Following his earlier experiments with rats, in 1972 Calhoun would later create his "Mortality-Inhibiting Environment for Mice": a 101-inch square cage for mice with food and water replenished to support any increase in population,[1] which took his experimental approach to its limits. In his most famous experiment in the series, "Universe 25", population peaked at 2,200 mice and thereafter exhibited a variety of abnormal, often destructive behaviors. By the 600th day, the population was on its way to extinction.[2]