3 Days after People: Rain begin building up in the Chicago River, one of the most engineered rivers in the world. With no humans to regulate the water level, downtown Chicago is flooded and the bottom floors of the Sears Tower fill up with water.
15 Years after People: The glass planes of the Sears Tower begin to fall out and crash to the street. As the steel frames around the glass rusts, it expands and puts huge pressure on the window, pushing it out of the frame.
200 Years after People: Corrosion has devastated the sears Tower, leaving it as a hollowed-out honey-combed husk. The building's 72 elevators all stopping at different floors are now crashing through the building and weakening its floors as the cables holding them break. The elevators security brakes kick in, but they corrode, too, and stop working. One of the two elevators connecting the ground floor with the observation deck falls nearly 1000 feet to the ground. But the onslaught of nature will need one more sledgehammer to topple the massive, awesome landmark.
The Willis Tower has its final stand. The water that flooded the basement of the Willis Tower 200 years ago has been coroding away the steel and concrete bars that holds the structure up. Soon, the enitre tower collapses, destroying everything in close range to it.