Conquer 75 glacial hills on Wisconsin’s ultimate wilderness singletrack.

The Thunderdown in the Underdown, happening August 16, 2025, is a grassroots mountain bike challenge in Merrill, Wisconsin, and part of the Wisconsin Endurance Mountain Bike Series (WEMS). Riders traverse a demanding 37-mile loop carved by glaciers, packed with over 75 hills, rugged singletrack, remote lakes, and wooden boardwalks. Expect a true backcountry experience—this is a test of both physical and mental grit!

The race features a single 37-mile lap on technical singletrack formed by glacial activity, with steep climbs, descents, and multiple boardwalk crossings.

• True wilderness immersion with minimal cell service
• Live music, food, and beer at the post-race party
• Custom finisher mugs for all finishers
• Option for 111-mile (3 lap) ultra-endurance ride
• Remote lakes and scenic glacial landscapes