From the Wikipedia page (http://en.wikipedia.org/wiki/Ultra-Trail_du_Mont-Blanc):
The Ultra-Trail du Mont-Blanc (also referred to as UTMB) is a mountain ultramarathon. It takes place once a year in the Alps, across France, Italy and Switzerland. The distance is approximately 166 km (~103mi), with a total elevation gain of around 9,400 m. Some believe that it is the most difficult foot race in Europe.
While the best trailers complete the loop in slightly more than 20 hours, most runners take 30 to 45 hours to reach the finish line.
On the bucket list for sure. Must focus on finishing http://RaceTheTrace.com first!!!