Although it is devastating to find out that Chicago is sinking, we’re only talking one millimeter a year. That’s four inches in a century, but only until the land in North America brings itself back into equilibrium.
When glaciers covered Canada and the northern Great Lakes tens of thousands of years ago, their weight pushed down on the Earth’s surface and caused it to sag. And this pressure forced the Earth’s mantle, the layer of hot, plastic rock that’s located beneath the Earth’s crust, to give way and ooze out and sideways. It’s like the Earth is a giant waterbed. When you lie down on it, the surface of the mattress under you sinks, forcing the water into other parts of the mattress.
In the twelve thousand years since the glaciers receded, the Earth’s crust has been slowly returning to its original shape. So the land in Canada and the northern Great Lakes is slowly rising, while the rest of North America is slowly sinking, like a seesaw, enabling the mantle to flow back north. And as the Lake Michigan floor north of Green Bay rises, all that water sloshes south towards Chicago.