it is a 10 K (6 miles) race in Boulder, Colorado, that attracts up to 50'000 runners/walkers every year... afterwards there is an elite race where the winner of the male and female race take home a cash prize of 20'000. It's a wickedly fun event where they have bands along the race course and lots of spectators cheering you on...
I hope I did it justice, explaining the Bolder Boulder