The bluish cave, which varies in shape each year, has a rounded ceiling made of thick ice about 5 meters high, and is about 20 meters long.
The tunnel is accessible by foot in 15 minutes from the chairlift at Glacier 3000 above the resort of Les Diablerets, but at your own risk, organizers said.
The natural cave, also known as "The Mill," forms through a siphon effect. Each spring and summer, the cavity fills with water from the snow melt, forming a lake. In autumn, the plug disappears and the water drains, leaving the cave.