On an October night in the Sichuan mountains, astrophotographer Shengyu Li set up his camera to record the tranquil beauty of star trails over Mount Xiannairi. What he captured instead was something entirely unexpected. A surprise avalanche unfolded right in front of Li’s camera. Luckily, our photographer was far from any danger, but the chance encounter also captured a very rare physical phenomenon: a mysterious cascade of blue flashes.

The event, described as both “thrilling and intriguing” by Li, occurred when an icy serac — an unstable block of ice — broke away from a glacier and hurtled down the mountain. As it fragmented, the tumbling ice blocks emitted bursts of blue light. These luminescent flashes were not visible to the naked eye during the event but appeared in Li’s timelapse footage.

Li’s discovery has baffled scientists and photographers alike. It’s not clear at all what happened, but the prevailing hypothesis is that the flashes are caused by a phenomenon known as triboluminescence.

Triboluminescence occurs when certain materials emit light as they are fractured, scratched, or rubbed.

Mountaineer Carson Reid, who analyzed the footage, told SpaceWeather.com that the flashes were most prominent at key “smash points” where the ice collided with the mountain. “The serac would have fragmented as it tumbled down and smashed into natural obstacles,” Reid explained. The violent collisions likely generated the conditions necessary for triboluminescence to occur.

What causes breaking ice to emit blue light?

As a side note, the extraordinary physicist Richard Feynman once spent a year teaching in Brazil. When it was time to leave, Feynman held a farewell lecture — but this wasn’t your typical, polite farewell. This was Feynman, after all. Feynman was upset by a perplexing problem: students were brimming with “book knowledge” but lacked a true understanding of the material.

When Feynman learned that senior academics and government officials would attend his lecture, he agreed — on the condition that he could speak freely. On the day of the lecture, staff grew uneasy when they saw him carrying an elementary physics textbook, written by a respected figure in the audience. “You’re not going to say anything bad about the textbook, are you?” someone asked nervously. “Everybody thinks it’s a good textbook.”

Halfway through the lecture, Feynman held the textbook up. “I have discovered something … by flipping the pages at random, and putting my finger in and reading the sentences on that page, I can show you what’s the matter …”

As Feynman stuck his fingers in the textbook at random, he started reading:

“‘Triboluminescence: Triboluminescence is the light emitted when crystals are crushed.’

“And there, have you got science? No! You have only told what a word means in terms of other words. You haven’t told anything about nature — what crystals produce light when you crush them, why they produce light. Did you see any student go home and try it? He can’t.

“But if, instead, you were to write, ‘When you take a lump of sugar and crush it with a pair of pliers in the dark, you can see a bluish flash. Some other crystals do that too. No one knows why. The phenomenon is called “triboluminescence”.’ Then someone will go home and try it. Then there’s an experience of nature.”

Physics caught on camera

Li’s footage isn’t the only instance of these mysterious blue lights. Another astrophotographer, Lu Miao, captured similar flashes three weeks earlier during an avalanche on Muztagh Ata, another mountain in China’s Xinjiang region. Once again, the phenomenon was discovered only upon reviewing the timelapse images.

Despite the mounting evidence, much about this phenomenon remains unexplained. Although triboluminescence is well-documented in laboratory settings, its occurrence on this scale, involving massive ice structures, is unprecedented.

In an explainer on X, science communicator Erika wrote that triboluminescence “occurs due to the breaking of chemical bonds or the sudden separation of surfaces, which can create electrical charges. These charges cause ionization of the surrounding air or excitation of the material itself, leading to visible light emission.”

She went on, adding that “the exact cause is not fully understood, but it’s often attributed to rapid charge separation and recombination, which produces a burst of light. When ice breaks apart, friction or shock can release similar flashes. In the case of avalanches, the intense fragmentation and collision of ice could explain the observed blue flashes.”

For now, the blue lights remain an enigma. Li and others hope that further research, or even more accidental recordings, might shed light — quite literally — on this extraordinary natural event.