Leadville Trail 100 Ultramarathon (aka The Race Across The Sky or the LT100) is an ultramarathon

Ultramarathon

An ultramarathon is any sporting event involving running longer than the traditional marathon length of .There are two types of ultramarathon events: those that cover a specified distance, and events that take place during specified time...

 held annually on trails and dirt roads near Leadville, Colorado

Leadville, Colorado

Leadville is a Statutory City that is the county seat of, and the only municipality in, Lake County, Colorado, United States. Situated at an elevation of , Leadville is the highest incorporated city and the second highest incorporated municipality in the United States...

, through the heart of the Rocky Mountains

Rocky Mountains

The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...

. First run in 1983, runners in the race climb and descend 15600 feet (4,754.9 m), with elevations ranging between 9,200-12,620 feet. It is common for less than half the starters to complete the race within the 30 hour time limit.
The course is a 50 miles (80.5 km) out-and-back dogleg, starting at 10200 feet (3,109 m).