One of the top attractions of Chamonix, you won't regret this 4 hour roundtrip hike up to one of the most beautiful alpine lakes the French alps has to offer at 2399 meters (7,870 feet) with an incredible panoramic view of Mont Blanc.
This adventure begins at the Flégère gondola in Les Praz (a village in the NorthEast section of Chamonix), which is easy to get to with the free Chamonix bus. Take the Flégère gondola to La Flégère, situated at 1894 meters (6,213 feet). The ride up itself shouldn't take more than ten minutes, but standing in line may take up to 40 minutes if it's very crowded during the summer, so I suggest going as early as possible. You can also hike up to La Flégère in lieu of taking the lift, which is a strenuous 832 meters (2,729 feet) hike.
The trailhead to Lac Blanc is clearly marked just beyond the exit of the La Flégère building. From this point, the hike up to Lac Blanc should take roughly two hours (one-way). Continue down the trail, which ascends and descends (but mostly ascends) until the lake. There are areas where the trail gets very steep and rocky, and (during the colder months) some areas of the trail get icy and slippery, so be sure to walk cautiously and take frequent breaks. The trail is generally pretty clearly marked. However, in moments of uncertainty, look out for trailhead signs to Lac Blanc and follow the cairns (stacks of rocks that people build as trail-markers).
Once you get to the lake, I highly recommend exploring the hills surrounding the lake off-trail to get a better view of the lake and the surrounding vastness of the alps.
You can return the same way or opt to hike all the way down back to Chamonix (roughly a 1000 meter (~3,000 feet descent).
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Enjoy!