Blue Hole
Estabrook Park was originally the site of the Milwaukee Cement Company.
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From 1870 to 1911, the company dug vertical shafts and horizontal tunnels underneath the park.
It also quarried limestone from below the river, which left two man-made lakes, Cement Lake and Blue Hole, tempting to swimmers, but dangerous.
Despite not being suitable for swimmers, Blue Hole offers a unique habitat that acts as an ecosystem for bacteria, bugs, fish, amphibians and birds. There also is documented presence of dragonflies and mussels.