Hi there. I'm Washington Post reporter Lillian Cunningham. Stay tuned after the show to hear about my latest podcast, Moonrise. It's the dark but true story of why we went to the moon and what we found there. The full series is available now.

Hey, history lovers.

I'm Mike Rosenwald with Retropod.

A show about the past.

Rediscovered.

50 years ago, a scientist named John Calhoun was working at the National Institute of Mental Health.

Calhoun was a research psychologist. And oh, did he love rats. Calhoun loved them so

much that one day, in 1947, he asked a neighbor if he could build a rat colony on a quarter

acre of vacant land behind his house. For some reason, the neighbor said, yes.

And so Calhoun built a massive rat city with room for as many as 5,000 rat citizens.

This rat city was the beginning of an experiment that revolutionized the way we see human behavior.

There are many oddities to founding a rat city, which Calhoun quickly discovered.

For instance, Calhoun was surprised to find that even though there was room for 5,000 rats,

the population, according to a 2009

journal of social history study, leveled off at a mere 150. He never could figure out why.

So Calhoun decided to tweak his rat city to create a rat paradise. Well, actually, a mouse paradise.

Yep, Calhoun pivoted to an entirely new creature.

From 1965 until 1973, Calhoun built and studied a mouse utopia.

He created mice apartments in towers that resembled high rises.

He took steps to improve the colony's hygiene and reduced disease,

just like, say, a health department director would in a human city.

Calhoun gave them all the food they could want, and the ample

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