The Finnish utility company called ‘Helen’, 100 % owned by the City of Helsinki, built Katri Vala, the largest heat pump plant in the world to produce heat and cooling. The Katri Vala Heat Pump Plant, located underground, recycles waste heat from purified wastewater, as well as excess heat from buildings such as data centres. The heat pump operates on the same principle as, for example, a refrigerator. A rock cave for the heating plant was excavated under the Katri Vala Park, a few kilometres from Helsinki city centre, ideal due to the fact that an outflow tunnel for purified wastewater and a multi-utility tunnel intersects under the park. Through this system heat and cooling energy produced at the plant is transmitted to customers. The technology is therefore able to utilise heat energy that would otherwise go unused. 2017 saw production increase to a total of 570,000 MWh, 8% of the heating needed for the city. Givern its success, a sixth heat pump will be commissioned in 2021 and will enable the plant’s production volume to increase by up to 30% compared to 2017. So far, the carbon dioxide emissions of the plant are 80% smaller than for example in separate heat production with heavy fuel oil.