Estabrook Park was originally the site of the Milwaukee Cement Company.
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.