These natural caves, formed underneath a large plateau opposite downtown St. Paul, have been host to numerous activities. Used by native tribes before settlers arrived in the area, in the 1840's natural silica was mined from the cave for glass making. A mushroom farm was started in the caves by a french family in the early 1900s, who later turned the location into a speakeasy during the days of prohibition, naturally attracting less-than-legal business dealings, and even a reported mob killing.
The caves were turned into a disco in the 70s, and today continue to be an event spot. The caves are, of course, also purported to be haunted by murdered gangsters and speakeasy patrons. Cave tours are conducted on weekends.
Obscura Day location: April 9, 2011.