This small lake outside Stockholm, Sweden, emits otherworldly sounds as Mårten Ajne skates over its precariously thin, black ice. “Wild ice skating,” or “Nordic skating,” is both an art and a science. A skater seeks out the thinnest, most pristine black ice possible—both for its smoothness and for its high-pitched, laser-like sounds.

File under: Do not try this without experienced wild ice skaters at your side. Mathematician and author Mårten Ajne loves the challenge of long-distance skating on thin, untouched ice, and has written books on the physics and culture of the sport, including safety tips for how to avoid dangerous ice. How does he determine if the black ice is strong enough to bear his weight?

With experience you get better and better at it, but you’re never certain. Once in a while you might fall, and take a plunge.

But we always go in packs for safety. It’s quite a social sport, actually… It’s not like waking up in the morning and saying, “Hey it’s a nice day, let’s go skating.” It’s much more complicated than that.

It does involve a lot of math—it’s the temperature, the atmospheric conditions, it’s a lot of things. How long it will take for this lake to cool down and for it to freeze… We look at satellite images and the smoothness of the surface, all to work out if we can go.

This beautiful footage was filmed by ice skating enthusiast and filmmaker Henrik Trygg. Here’s his original film sans annotations: The Sound of Ice.

And this six minutes of footage was filmed by Anje himself as he skates with friends “on new brackish thin ice along Ramön, Stockholm archipelago…”

Learn more about how wild ice skating at National Geographic: How Skating on Thin Ice Creates Laser-Like Sounds.

Next: Why does a frozen lake sound like a Star Wars blaster? Plus: Pow Surf 101, ice skating on Amsterdam canals, and Ice Drumming on Lake Baikal.

h/t Joss Fong.

See more videos about...