Chicago stretches for 29 miles along the southwest shore of Lake Michigan in Illinois. The area of most interest to visitors is the downtown area that has grown up to the north and south of the Chicago River. This river naturally flowed into Lake Michigan, but the city government reversed its flow in 1900 to improve sanitation. Just south of the point where the Chicago River and Lake Michigan meet is the Loop. The Loop derives its name from the fact that all of Chicago's elevated railroad ("El") lines converge here, and the Brown, Purple, and Yellow lines follow a looping path encompassing about 35 square blocks. Grant Park is between the Loop and Lake Michigan.
A few blocks to the west of the top of the Loop, the Chicago River splits into a North Branch and a South Branch. Wacker Drive runs a parallel course to the Chicago River and its South Branch. Looking at a map will make this perfectly intelligible; but for now, just know that you may run up against Wacker Drive and the Chicago River in places that you don't expect. The Sears Tower stands in the south-western part of the Loop (2 blocks west of the El), between West Adams Street and West Jackson Boulevard. You can't miss it!