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From WBEZ.

Hey, Jessica Popovac here.

A few weeks ago, we ran a story about early nuclear experiments at the University of Chicago during World War II, and whether or not Chicago's mayor and government even knew they were happening.

We were not to reveal anything of what you do. Don't take up with strangers. If you're having a sandwich someplace or a beer or whatever,

watch out that people who may engage you in conversation, they may actually be the enemy.

It reminded us that back in 2013, Curious City actually did a fascinating story about that same experiment.

The question came from Chicago native Mark Eiffert.

The first controlled nuclear reaction took place under University of Chicago's

Stag Field. And I'm wondering if that site is still radioactive. For an answer, we turned to

audio producer Katie Mingle. Now, to really unpack this question, it's helpful to know the full

backstory. And it's a good story, so let's

start back at World War II. American scientists were running experiments to help win the war,

and some of that work was done in Chicago. To help tell the story, I found a Pulitzer Prize-winning

historian who wrote the book about this. I'm Richard Rhodes.

As Rhodes tells it, in 1938, scientists in Nazi Germany had discovered that when they

split a uranium atom, that atom would release neutrons.

Those neutrons could go find more uranium atoms to split, which would release even more

neutrons, and you'd have a chain reaction.

Once the discovery was made, it was very quickly clear to scientists that this new extraordinarily

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