Underdown Trail Races
Merrill, United States • 15 Aug 2026
Underdown Trail Races
The Underdown Trail Races are grassroots trail runs and mountain bike races. They take place in Merrill, Wisconsin on the Underdown Trail system. The location is 6 miles off Highway 51.
The running event offers three distances: 60k, Marathon, and half marathon. The mountain bike event, called Thunderdown in the Underdown, offers distances of 111 mile, 37 mile, and 16 mile. The bike race is part of the Wisconsin Endurance Mountain Biking Series (WEMS).
The course consists of backcountry singletrack trails formed by glaciers. The terrain includes many hills and dozens of boardwalks, and passes by isolated lakes. The event notes the trails go through remote areas with spotty cell phone service.
With registration, participants receive a well-marked course, stocked aid stations, and drop bag transport. Finishers get a custom commemorative mug. There is an after-race party with live music, food, and beer. Proceeds from the event go toward building and maintaining trails at the Underdown. Visit the organization's website for the most recent information.