“Underground City XXI” has as main goal the rehabilitation of a real ex-coal mine located in Labin and Rasa (Region of Istria, Republic of Croatia) which was closed in 1998 ending the main industrial activity of the region. “Underground City XXI” aims to create a real underground city in these closed coal mines. The project has already developed many concrete steps for a future start of the realization of the underground town, as for example, the creation of Cultural Centre “Lamparna” (future entrance of the real “Underground City”) in September 1998, the technical expertise, which proved technical feasibility of the project, in 2000, the laser measuring of the coalmine, in 2001, or the general Feasibility Study proving the economic feasibility of the project, in 2002.