The longest off-piste ski trail in the world is Vallee Blanche in Chamonix, France.
Vallée Blanche is an incredible 14.2 mile descent covering a massive 9000+ feet drop from the Aiguille du Midi back to Chamonix. While it may not be the steepest terrain available in Chamonix, there are considerable risks including hidden crevasses and serious avalanche risks. Taking these hazards into account, hiring a local guide is highly recommended if you are considering skiing Vallee Blanche.
Take a trip from the top of Chamonix including the tricky walk down to the run (some consider this the sketchiest section) and stopover for some lunch and glass of wine alpine hut that seems like its out of a storybook. Skiing in France is unlike anywhere else in the world.