‘Untertage’ is the expression for miners that are going down for work. The owners of the Boiler gay sauna took it as the theme for an expansion of their spaces.
Untertage has iron cages for example that usually take miners down under the ground. The place, in a basement in an inner courtyard, has a cosy feeling and has party nights independent from the sauna. On busy sauna days though it is the bar of the Boiler.