The Underdown is a gem of a trail system that has extensively changed from a handful of years ago. If you are looking for a rugged back country style ride with lots of technical singletrack with very little doubletrack networking, this is for you. There are now two trail heads, the main north trail head located off of Copper Lake Road northeast from the City of Merrill, nearest to Lincoln Hills correctional facility. Straight out of the north parking lot into the woods you hit singletrack that combines climbing with equal length downhill shots. For some 1x1 riders you might find yourself walking most of the climbs if you're not ready for them. Be prepared for some loose rocks, off camber runs with switchbacks, wet root sections and sweet downhills. There are also a number of nice bridges, boardwalk features along the trail. I've found that the knobbier the tire, the better. Updates in 2017 have brought a bit of machine built reconstruction to some of the interior trails to add a bit of flavor to the entire system.
The second southern trail head is off of County Hwy R/Heineman Road in the City of Merrill Memorial Forest. The trail head at this time is nothing more than a grassy parking area and has NO drinking water or restrooms so be prepared. Head south around the metal gate for about 1/4 mile and you will access the Prairie Dells loop. A fun, easy 5 mile loop that highlights itself at the Prairie River Dells overlook about halfway through the loop. This trail is primarily machine constructed single track with a touch of double track. Plenty of climbing but not as severe as the northern Underdown area and all of the climbs are rewarded with fast, flowing downhills and berms.
Heading north from the Prairie Dells trail head you can link up to the Underdown trails via the Connector trail, another machine built gem with lots of good flow, berms and some jumps. Access the connector trail from a double track entrance on the north side of the road right where the County Hwy R pavement ends into gravel road. Scoot around a cable gate and the single track shoots up the first ridge. Heading north for the next 2.3 miles you will primarily climb with a few short downhill rewards. But after you have ridden some loops on the Underdown interior head back down the connector for some really fun longer downhills with only short uphill sections.
For a big day in the saddle combine the main outer perimeter of the Underdown with the Prairie Dells loop and you have about 30 miles of riding.
Overall the singletrack trail system is perfectly maintained (even in the fall) and is marked very well.
For more trail info check out: http://thunderdown12.blogspot.com/