Don: Yael, I'm going to take some vacation time so I can visit Chicago before it sinks and disappears. I mean, I'd like to see the Sears Tower and eat some deep-dish pizza before it goes.

Yael: Relax, Don. Although Chicago is sinking, we're only talking one millimeter a year--D: But that's four inches in a century!

Y: But only until the land in North America brings itself back into equilibrium.

D: What do you mean?

Y: Well, 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.

D: Oh! 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.

Y: You have a water bed?

D: But of course.

Y: Anyway. 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.

D: Kind of like tilting a bathtub.

Y: Yep.

D: Hmm! So even if I don't have to rush out for pizza, it does sound like an issue from a Great Lakes management perspective.

Y: It sure is.