Dark Mountain is the oldest of all the trail systems at the lake, and was designated a National Recreation Trail in 2005.
There are several numbered and named small loops at Dark Mountain, all intersecting with a 2-track "Core" trail which goes through the whole system. Follow the BLUE arrows for the all-encompassing route (mapped here), including the "Snake" and "Lake" loops that each contain a solid climb and descent.
From the trailhead, the climb up the entrance trail takes you through the first-built trails at the lake. After about a mile, you'll get through a very rooty section and begin a descent that winds down "The Snake" to another section of Fish Dam Creek waterway. Enjoy it, because as soon as you see the stream, you'll be begin a sustained 1 mile climb out.
After about 3-4 miles in, the trail begins the 200 foot descent of the "Lake Loop." A fantastic finale awaits after the climb back to the top of Dark Mountain, the exit trail. This section has hosted Downhill and Super-D events, and contains optional lines with big banked turns and jumps.