Maridalen is a valley in the north of Oslo, Norway, just above the suburb of Kjelsås. Considering that Maridalen is within the Oslo city limits, this area has a very low population density. The valley consists of forests, lakes and agricultural land. Most of the valley is made up of the lake Maridalsvannet, which serves as the primary source of drinking water for 90% of Oslo's population. The only hydroelectric powerplant within the City of Oslo, Hammeren Hydroelectric Power Station is located at Brekke in Maridalen.
https://en.wikipedia.org/wiki/Maridalen