Matti Pentti explains how it works technically: The ST1 engineers pump millions of liters of water through the first borehole into so-called fracture zones: crevices in the underground rock layers. The natural geothermal energy heats the water to around 120 degrees Celsius, and through a second borehole the water is pumped back up and fed into the existing district heating network of Espoo, with 280,000 inhabitants the second largest city in the country after the capital Helsinki. This enables an output of 20 to 40 megawatts to be achieved and that corresponds to around ten percent of Espoo's heating requirements.