John Bumpass Calhoun was a researcher at the the National Institute of Mental Health beginning in 1954. He had spent years already studying the behavior of groups of mice kept in captivity. Now he had space and resources to build mouse utopias. And he did, ever bigger and better. His largest experiment was a room sized mouse universe in which he placed eight mice in 1968. They thrived quickly due to the many amenities: plenty of food and water, nesting boxes, room to run, and most important for a mouse, safety from predators. Soon, they began to have families.
This is a far cry from a wild mouse's life—no cats, no traps, no long winters. It's even better than your average lab mouse's, which is constantly interrupted by white-coated humans with scalpels or syringes. The residents of Universe 25 were mostly left alone, save for one man who would peer at them from above, and his team of similarly interested assistants. They must have thought they were the luckiest mice in the world. They couldn't have known the truth: that within a few years, they and their descendants would all be dead.
Calhoun saw the results of this population growth experiment echo his earlier, smaller mouse utopias. Read about Calhoun’s experiments and how he inspired a book called Ms. Frisby and the Rats of NIMH, which was later made into a movie called The Secret of NIMH, at Atlas Obscura. There's also a video that further explains the experiment.