The Illinois-Indiana state boundary marker, one of the oldest landmarks in the Chicagoland area, was constructed in 1838 under the direction of the United States Surveyor General to commemorate the establishment of the common boundary between Indiana and Illinois. It stands 159.359 miles due north from the banks of the Wabash River. When the original survey was done in 1821, a pine post marked the boundary. Built of large limestone blocks in the shape of an obelisk, it stands over 15 and 1/2 feet tall and weighs over 38,000 lbs. The obelisk was originally located approximately 191 feet south of its current site. In 1988, Allen J. Benson, an executive of Commonwealth Edison, worked with local preservation groups to reach an agreement under which the power plant paid for the obelisk's refurbishment and relocation. In 2021, Hammond Mayor Thomas M. McDermott, Jr., the Hammond Port Authority, and 10th Ward Alderwoman Susan Sadlowski-Garza created the plaza around it.