In early medieval times it was seen as a major obstacle in land travel between the regions, and so the Baltic Sea was largely used instead. Together with the extensive forests of Tiveden and Tylöskog to its west, Kolmården formed the crucial and wartorn frontier between Svealand to the north and Götaland to the south during the formation of the Swedish nation in the 13th century.
In the Icelandic text Sögubrot af nokkrum fornkonungum, Kolmården between Svealand and Östergötland is described as the border between Sweden and Östergötland (..Kolmerkr, er skilr Svíþjóð ok Eystra-Gautland). It also appears as Mirkwood in some editions of Sögubrot, in the legend of Helgi Hundingsbane and probably also in Völundarkviða.
Today the forest is traversed by the Norrköping–Nyköping railway, the Norrköping–Katrineholm railway, and the E4 highway, all connecting southern Sweden with Stockholm and the Mälaren region. Along the former there is also a Kolmården railway station, at the village of Krokek by Bråviken.
Kolmården Wildlife Park 
The Kolmården Wildlife Park, opened in 1965, overlooks Bråviken bay. This zoo includes the only dolphinarium in Sweden.
See also 
Media related to Kolmården at Wikimedia Commons
- www.Kolmarden.nu partially in English - Where to eat & live when visiting Kolmarden - Independent free private site