As far as I know it was a rock quarry/gravel pit.
30 years ago I could still find foundations of the military presence but while there are still some concrete markers to be found, most every thing else has been swallowed by the forest.
The CCC camp there was in operation up until the start of the war. The Army Signal Corps took it over as part of a radar training program. For years there were rumors of it being a top secret camp, but that doesn't appear to be accurate. The training program involved civilian students who studied radio and electronics at several colleges in Chicago and then went to McDowell to train on older equipment. Upon graduation the students went into the signal corps. A few years ago it was discovered that after the radar school shut down the Office of Strategic Services ran a communications school at McDowell because their main school near Washington, DC was full. It does appear that the OSS kept their operation secret and even used the radar school as a cover. The forest preserve district has more info on it and sometimes does programs about the history of McDowell.
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