I rode Underdown last year an had trouble finding info but the trails are marked really well and I had no trouble out there. Trails are pretty sick. I had the place to myself too. I'd love to get back there ASAP. Watch out for them rocks, they sneak up on ya.
Indeed. It's a bit of a drive, but worth it if you are looking for something different. It's very well marked, but more on the slower side of things (at least for me). More technical, kind of up and down riding. Make sure you bring bug spray, mosquitos there do not give up.
Last year a friend and I left Chicago early morning and rode Levis Mounds one day, and then headed over to camp nearby Underdown. We rode that the second day.
We used fattireguides.com as our guide. That site is top notch.