Not only “the mole people” live underground, as there are large urban areas, publicly accessible, built below the ground level, and indoors in general.

As an example of an indoor world: Easton Helsinki, in Finland, which has about 4.8 million visitors per annum. (Source.) It’s built on multiple levels. OpenStreetMap doesn’t yet have support for such multi-levelled maps, but it could: a level is just a layer.

Mostly underground is the bus terminal at Kamppi, in Helsinki. It has elevators and escalators carrying countless people up and down, in and out of tunnels and halls. Most of this world underground isn’t yet seen in OpenStreetMap.