The Illinois–Indiana State Line Boundary Marker is a limestone obelisk that commemorates the establishment of the border between Illinois and Indiana.

The obelisk is composed of large limestone blocks, standing more than 15½ feet (4.7 meters) and weighing more than 38,000 pounds.

By the 1980s, however, due to landfill related to the area’s industrial development, the shoreline had shifted several hundred feet north, and the obelisk was hard to find and damaged by graffiti in a remote wooded location.

In 1988, thanks to local conservation efforts, it was moved to a safer, more easily accessible location next to the guard house for the State Line Generating Plant.

Then in 2021, Hammond and Chicago collaborated to build a small park, called State Line Plaza.