It looks like a forest from the street. But when you get into the forest it opens up. Its extemely large. You'll see many trails on the ground. The best time to go to those trails is at night or the morning. The trails are mostly rocky and muddy.
Starting from SE 8th ST drive west till you get to SW 137th AVE. Turn right. Drive north on SW 137th AVE. On the right of SW 137th AVE is the trails. There are many entrances to the trails. The trails begin right when you enter SW 137th AVE.
http://www.singletracks.com/item.php?c=1&i=1039