Caves of St. Louis
St. Louis is built upon a complex of natural caves which were once used for the lagering of beer by early German brewers. Underground caves are naturally cool, which was especially attractive to brewers before the advent of refrigeration.
Several breweries were built atop these natural caves, which were altered to suit their purposes. Stone arches and brick ceilings prevented water seepage and uneven cave floors were paved with brick. In addition to being used for the storage and lagering of beer, such naturally cool places were sometimes employed as beer gardens, places for entertainment.
The "Cherokee Cave" was one attempt to turn the caves near the Lemp Brewery into a commercial attraction. These caves were closed and the site demolished to make way for the Interstate 55 construction. Some parts of these caves still remain, but access is restricted to the public.
Old caves were often filled with rubble from the buildings above them when they were demolished. Therefore, there may be many homes and houses currently built above entrances to the old system of caves which have since been filled in with rubble.
One good book for further reading on the topic is "The Lost Caves of St. Louis." (see link below for online version)