Chicago history comes in layers.

Thinking about the spot in southern Grant Park from which Barack Obama delivered his 2008 presidential victory speech, Geoffrey Baer realized that, four decades earlier, the Democratic National Convention protesters had gathered in that same spot.

Before that, airplanes took off and landed from there during a deadly 1911 air show, and before that, it was part of the lake, accepting Great Chicago Fire rubble to become parkland.

"Chicago Time Machine," Baer's 20th special on area history and neighborhoods for WTTW-Ch. 11 (7:30 p.m. Tuesday), explores these connections and many more, demonstrating extraordinary overlapping in the city and, at the same time, its lack of sentimentality about places of import....