It's Curious City, where we take your questions about Chicago and the region, and investigate, report, explore, from WBEZ.

Hi, I'm Chris Bentley, a reporter with Curious City. And I'm Jen Mason-Garb of the Chicago Architecture Foundation.

Today we're teaming up on a question we're both geeked about. Yeah, it jumped out at me when I read through the hundreds of curious city questions about architecture. Me too. I'm a casual geology nerd, and I've actually got a geologic time scale pinned to my wall. Wow. Okay. Well, then let's get started. Okay. So our question asker is Mike Vendell, and he works as a programmer in Chicago's Loop.

Mike doesn't usually give much thought to how skyscrapers stay standing.

But when I'm outside enjoying the lakefront at the beaches or other park areas, you know, you see the sand and you see these huge skyscrapers in the skyline, and you think, like, you know, how do they stay stable in that structure?

So he asked Curiosity how it all came to be.

I want to know how these huge, massive buildings are stabilized in sandy and swampy soil.

It took a lot of experimenting and some feats of engineering to deal with a swampy

soil that Mike refers to. So it makes sense to clear up what exactly are soils made of.

We put the question to Ray Wiggers, a geologist at Oakton Community College.

If you can imagine your front yard at a little muddy spot you have,

after it's rain for a while, and that sediment is really, really saturated, and it's very

oozy. Can imagine trying to build a skyscraper in that? Wiggers says a giant prehistoric lake

used to cover downtown Chicago, and all the ancient organic matter, all that soil, the dead leaves,

the dead fish sunk to the bottom and became

layers of sand, silt, and clay. That's what makes up our soil today. But underneath the mucky

layers is the jackpot, a rock called dolomite. That's our regional bedrock, sort of a very hard,

solid, bone, white rock. Dolomite is the bedrock that can anchor even the biggest skyscrapers.

But in Chicago, it's far underground, which is a problem for the skyscraper business.

You have to either dig through all that glop, all those wet sediments going down sometimes as far as 80 feet or so.

Or you have to have some kind of a structure that will float in that sediment and hopefully keep the

building, you know, standing up the way it should.

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