Forest Park is the largest City Park in the world. Although it is within city limits, it is a wilderness area with natural flora and fauna ranging from squirrels all the way up to black bears. The park is on the east side of the Portland Hills, and is a kilometer or so wide, and about a dozen kilometers long, running from Northwest Portland almost up to Sauvie Island.
The land of Forest Park is only a short distance outside of downtown Portland, but it had for the most part never been built on due to the steepness of the hills. In 1946, the area was officially designated as a park, and has remained so since, Immediatly to the south of it, and on its eastern side, lies the West Hills, home of some of Portland's most beautiful, and most expensive, real estate. However, despite the great financial rewards possibly involved, there is little chance that Forest Park will be developed in the near future.
Forest Park is a good place to hike, and even after hours of hiking, you will likely get only a small fraction of the way into the forest, due to the winding nature of the trails. There are also plenty of side trails splitting off to the sides. The major trail that runs to the north is Wildwood Trail, which actually begins in Washington Park, climbs to the Pittock Mansion, and then proceeds into the forest itself, where it runs a jagged course northwords, criss-crossed by the east-west trails, that go to the top of the Portland hills. Parallel to it, but further down, is the Leif Erickson trail, a bike trail that is less steep and less winding.