Underdown Trail Races
Merrill, United States • 15 Aug 2026
Underdown Trail Races
The Underdown Trail Races are a series of running and mountain biking events held in Merrill, Wisconsin. The event takes place within the Underdown Trail system, which features terrain formed by historical glacial activity. The course is primarily composed of singletrack trails that navigate through remote wilderness areas, including passage by isolated lakes and dozens of boardwalks.
The trail run event includes the following distances:
- 60k
- Marathon
- Half marathon
The mountain bike portion, known as the Thunderdown in the Underdown, offers the following distances:
- 111 mile
- 37 mile
- 16 mile
The terrain is characterized by a significant number of hills, with some configurations featuring up to 75 climbs. Participants navigate backcountry singletrack that is well-marked but remote, where cell phone service is limited.
Entries include access to stocked aid stations, drop bag transport, and digital downloads of race photos. Finishers receive a commemorative mug and have access to divisional awards. The event also hosts a post-race gathering featuring food, beverages, and live music. Participation in the mountain bike races contributes to the Wisconsin Endurance Mountain Biking Series (WEMS). Proceeds from the races are used to build and maintain the trails at the Underdown. Visit the organization's website for the most recent information.