Oslo's richest wetland and bird reserve area, just five kilometres from the city centre and easily accessed by metro. Walk around the lake The graveled/paved trail around the lake is 4.5 kilometres (2.8 miles) long and is connected to a large network of trails in the Oslo forests. It takes less than an hour to walk around the water. Trails that start along the path can take you in the direction of Østmarka via Skullerud in the southeast or Svartdalsparken via Alnavassdraget in the northwest. Biodiversity The many natural habitats around Østensjø has given the area great biodiversity. In and around the lake, observers have counted 228 bird species and more than 20 mammals, as well as salamanders, frogs and lizards. Facts about Østensjøvannet lake Area: 0.29 square kilometres Length: 1770 metres Width: 260 metres The lake has been protected since 1992, and the area became an environmental park in 2002. In 2012, the site was authorised as a national wetland centre.