Leadville Trail 100 - AKA "Race Across the Sky"
This is the Super Bowl of mountain biking in the United States - 100 miles of high-altitude single track. You can't just register and ride, there are a number of coveted spots in the race and the selection process is grueling. There's a 4 day camp in July that involves riding the course first with past winners and biking legends, which also guarantees you a slot in the race. See link for more details.