Out of all the possibilities to heat and cool our homes, perhaps one of the most sustainable one is hidden under the ground. Geothermal energy could also be a greener alternative to air-conditioning and heating in historic buildings and museums. However, the efficiency of these systems is still low. This is the challenge that researchers of the CHEAP-GSHPs EU project are trying to tackle. We travelled to Croatia to interview different members of this project and saw first-hand the technology installed at the Zagreb’s museum of science.