In 2005, five Colorado firefighters climbed the equivalent of 110 floors in the stairwells of 1999 Broadway.

They were among the first to try this new way of honoring the first responders killed in the September 11th attacks, according to the Association of Memorial Stair Climbs.

By running the stairs in their full protective gear, they hoped to get some understanding of the selfless courage that drove hundreds of firefighters and other responders up the stairs of the World Trade Center towers, toward fires burning 80 and 90 stories above the ground. More than 400 first responders died in Manhattan that day, 2,000 were injured, and tens of thousands more were exposed to toxic dust.

Denver may have been the second city to host a memorial climb, according to the association. The association traces the tradition to Afghanistan in 2003, where a firefighter ran a two-story building 55 times in his military gear. He reportedly brought the tradition home to Albuquerque in 2004.

Now the events happen everywhere. The Denver event in particular has become a national model, as it was the first to support the National Fallen Firefighters Foundation.

A decade later, the climb has moved from downtown Denver to Red Rocks, where some 2,500 people were expected to attend on Sunday. Last year’s event raised more than $70,000.