Mont Blanc in France is the highest mountain in the Alps and the highest in Western Europe. Its snowcapped peaks can be seen for miles around, even when the ground temperatures are smashing records across the continent.

However, if you’re thinking of taking refuge up in these mountains to keep cool as the record-breaking heatwave slowly abates, then we should warn you, it may not be as cool as you think up there. Although if you were around at the end of June, you could have taken a nice swim in a beautiful lake that popped up at over 3,000 meters (9,800 feet) altitude – one that admittedly is not usually there.

Spotted and photographed by mountaineer Bryan Mestre on June 28, the lake formed at the base of the Dent du Géant mountain in the Mont Blanc massif and the Aiguilles Marbrées peak at around 3,400 meters (11,100 feet) altitude, and it caught him completely off guard.

Sharing his photo of the azure lake – and his shock at the discovery – online, he was contacted by another climber who shared his photo of the same area covered in snow just 10 days prior. June 28 saw France smash its previous highest-ever temperature record – 44.1°C (111.4°F) in 2003 – three times in one day, culminating in its all-time highest temperature of 45.9°C (114.6°F). The Mont Blanc region broke its own record the next day, reaching 9.3°C (48.7°F).

Speaking to IFLScience, Mestre described the conditions of the climb as “quite chaotic” as the heatwave was affecting different parts of the mountain range at different altitudes.

“In Chamonix, the city at the foot of Mount Blanc (1,000 meters up), they recorded temperatures nearing 40°C. In the same week, the isotherm (freezing level) rose up above the 4,700-meter line for several days. Normally, it hovers around 3,000 meters,” Mestre said in an email.  

“Needless to say, the lake was a real surprise. It's located in the 3,400 to 3,500-meter 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.”

The lake was around 10 meters by 30 meters (33 feet by 98 feet), holding around a few thousand cubic meters of clear meltwater, according to National Geographic France. Although many snowy mountainsides host lakes, it's unusual at such an altitude above sea level. Mestre said he and the mountain guide with him that day had never seen this before.

"I have been up there a fair amount of times, in June, July and even August, and I have never seen liquid water up there, and I'm not only talking about Mt Blanc massif," Mestre said. 

"I have been up dozens of [mountains] around the Alps. During the hottest days, you may find snow melting, but that's it."

A lake has appeared here before during a previous heatwave in 2015, observed by glaciologist Ludovic Ravanel, though it was not as big. It was likely linked to global warming, he told Humanity in an interview in 2018, as it was already being observed back them that scorching summers and rising temperatures at high altitudes are shattering parts of the Alps. 

Mestre described what he thought had happened as similar to a solar oven, the high temperatures allowing some of the glacier to melt, which revealed more of the rock. The dark rock then absorbs more heat, melting snow until the lake formed. Because the mountain's isotherm was so high up in June, the lake didn't freeze and disappear overnight.

Though beautiful to look at, it's not a good sign of things to come.