Following on Bob M's post, the UTMB thing is a lot of fun. I did the CCC variant, which runs from Courmayeur in Italy, via Champex in Switzerland to Chamonix in France. The part in the day runs takes you along the southern edge of val Ferret with amazing views north across the valley of the Mt. Blanc massif. The downsides are that there are so many people that it's hard to set one's own pace in the early parts on single file paths. Also, the organizers are a bit overzealous with the regulations – the rule book runs to about 10 pages and talks of spot checks for bag weight as well as urine and blood tests. I didn't see anything like that – but one of the guys I'm sharing lodging with got DQ'd because he was in the medical tent for more than 10 minutes, which seems a bit harsh. Overall, the difficulty is a lot like the old HURT 100k (before it became 108k!). But the difference is that the uphill sections are relentless – taking about 3 hours each. As Bob mentioned, there is a huge attrition rate with people dropping out. The most fun part of the whole adventure was coming to train, as the huts run by climbing clubs are great places to stay, and you get to enjoy the food, wine and general ambience a lot better that way. Here's a couple of pics of me at the start and at the award ceremony.