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At a Glance The French climbing instructor discovered the lake at 11,100 feet.

Ten days earlier, the lake wasn't there.

It formed during a heat wave over much of Europe.

Hiking at 11,100 feet in the Mont Blanc mountain range in June, Bryan Mestre saw an "alarming" sight he never expected to find at that altitude.

There, high in the French Alps at the base of the Dent du Géant and the Aiguilles Marbrées, sat a small, crystal clear lake.

"Time to sound the alarm," Mestre wrote in an Instagram post comparing his photo of the lake with one taken in the same spot 10 days earlier. The second photo, made by fellow mountaineer Paul Todhunter, showed nothing but snow at the base of the mountains.

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"Only 10 days of extreme heat were enough to collapse, melt and form a lake at the base of the Dent du Géant and the Aiguilles Marbrées. That I know, this is the first time anything like that has ever happened," he wrote.

The lake is about 33 feet by 98 feet.

"This is truly alarming," Mestre said. "Glaciers all over the world are melting at an exponential speed."

Europe was in the midst of an intense heat wave in June. On June 28, the day Mestre discovered the lake, France set a new all-time record high of 114.6 degrees in Gallargues-le-Montueux in the south. At least 12 stations saw temperatures higher than the previous national record of 111.4 degrees that day. The next day, record temperatures were measured in the Mont Blanc region , up to 48.74 degrees.

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“Needless to say, the lake was a real surprise ," Mestre, a 24-year-old rock climbing instructor, told IFL Science. "It's located in the 3,400 to 3,500-meter [11,155 to 11483-feet] area. You're supposed to find ice and snow at this altitude, not liquid water. Most of the time when we stay for a day at this altitude, the water in our water bottles starts freezing.”

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/vertlakealps.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/vertlakealps.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/vertlakealps.jpg?v=ap&w=980&h=551 800w" > The small lake, 33 feet by 98 feet, formed in June at the base of the Dent du Géant and the Aiguilles Marbrées in the Mont Blanc massif in France. (Bryan Mestre/Instagram)

Mestre, who lives in Chambéry in southeast France, told the London Evening Standard, “I was surprised to see it - in the Alps above the 3,000-meter line, water is always supposed to be in a frozen state.

“I have seen similar events in the Andes or in the Rockies, but the ecosystem is a lot different there. Snow is permanent in the Alps above 3,000 meters — it's not supposed to melt. Of course, with the whole global warming deal, it does melt, but it doesn't get this big."

Glaciologist Ludovic Ravanel saw a lake in the same area during a heatwave in 2015, IFL Science reported.

Mestre told the Evening Standard the area had a recent snowstorm.

"So the good thing is, it froze over and it's sort of back to normal now, but we're expecting another heat wave next thing, so it'll probably just melt again – it's a permanent problem now," he said.