What a terrific adventure it was to ski the Mer de Glace. This is the glacier that starts at the Vallee Blanche.
The run starts at the top of the Aiguille du Midi, at 3800m and runs for 20 km. Sensational!
It is dangerous skiing as you have to ski over snow bridges that form over crevases. You ski through ice forests or seracs.
Be sure to go with a guide. Here's a useful link: http://www.ski-chamonix.net/vallee-blanche-chamonix.html
There's nothing quite like this adventure. Highly recommended.