The idiom “skate on thin ice” usually means: danger—stop what you’re doing, or else. However, skating on thin, black ice is exactly what Swedish ice skating enthusiasts Henrik Trygg and Mårten Ajne love to do.

To hit the ice when it has just begun to freeze, in its most pristine, dangerously thin state, is the ultimate thrill in “wild ice skating,” or “Nordic skating.” It’s the “holy grail,” says Trygg. (Learn about the similar sport of backcountry ice skating.)

A photographer and filmmaker based in Stockholm, Trygg has made an art of capturing both the clear, black appearance of the ice, and the laser-like symphony of sounds created when an ice skater’s bodyweight passes over it. Ajne is a mathematician who has written books on the art and science of Nordic skating, featuring Trygg’s photographs. Both men have been skating for decades.

For the video posted on this page, Trygg filmed Ajne skating on 45-millimeter-thick ice on Lissma Kvarnsjö, a lake outside Stockholm, Sweden.

How—and why—would one skate on such thin ice? Ajne explains here:

Why do you enjoy skating on thin, black ice?

The simple explanation is it’s the most efficient non-mechanical way to get around. It’s really leisurely, unlike running. But I think the main driver behind it, in a more philosophical sense, is the urge to push yourself, because it’s a challenge.

What is the difference in style between wild or natural ice skating, and skating on rinks?

In figure skating or hockey they always accelerate and break. We go like the speed skaters, but much more relaxed. We’re not in a hurry.

We travel vast distances in a day. The average would be 40 to 50 kilometers [25 to 30 miles] on a typical day, but 100 kilometers [60 miles] would not be unusual. If you have a tailwind, you can do even more than that.

What are the ideal ice conditions?

The most desirable condition is virgin black ice, when a lake has caught its first ice cover and it has grown just thick enough to bear your joy. In favorable weather conditions this will take two days after freeze-over.

Five centimeters [2 inches] is often the limit [to how thin it can be]. If you’re close to shore, you can go thinner, up to about 3.5 centimeters, before it breaks. That’s for fresh, cold black ice. Brackish ice, which contains salt, needs to be thicker and is more difficult to assess.

But why doesn’t the thin ice break when you skate on it?

You might expect it to break immediately, but it doesn’t, because there’s water underneath.

Think of an ancient Roman arch or dome—the sides support the top. It’s about the same with the ice. Unless the ice gets too thin. The math is horrible, so don’t try it.

Do you ever really know if it is totally safe?

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.

What is it like when you do fall in?

There are a few hundred plunges every year, and mostly people just fall in and quickly get out, go home and dry off. If you injure yourself at the same time or if you're alone and no one can help you, it can be very serious. But those occasions are rare.

But why not skate on safer, thicker ice?

Thick ice becomes as interesting as pavement. When too thick it becomes inelastic and voiceless, often stricken by cracks, it gets ridges from thermal expansion, and sooner or later snow will entomb the joy of skating. Of course, it is also visually less attractive.

So how do you prepare for skating on black ice?

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.

Who else skates on black ice?

Engineers, mapmakers, and mathematicians like myself are a bit over represented. Also a lot of doctors.

Is there a competition element?

Friendly competing goes on, but there are no championships or Olympics. Everyone tries to be the first on the ice every year. Or find a really nice spot. Or be daring and adventurous.

We have a non-profit organization with a website where we share information, Skridsko.net. It means skate.net. People post pictures, it’s a little bit like a social media platform, but also a rich resource for ice information. There are skaters from the Nordic countries, Holland, also North Americans. We try to make it possible for other people to get out there—it’s difficult enough!

Why is black ice so good at producing those amazing, laser-like sounds?

In the video, the sounds are created by me skating on it. There is a distinctive sonorous tone and the noise from cracks striking.

Black ice doesn’t expand and contract because it’s kept warm by the underlying water, even when it’s cold out. Isothermal would be the technical word for it—in a narrow temperature range.

The sonorous tone is the song of black ice, best heard (and recorded) from a short distance. The layman explanation would be that the tone is inversely related to the thickness of the ice. The thinner the ice, the higher the tone. Intriguingly, the ice is about to collapse at high C, the supposedly highest note of a soprano opera singer, for example in Puccini’s Turandot.

Doesn’t regular, thick ice make sounds without someone skating on it?

Yes, when it contracts with temperature variations. It typically contracts at night and expands when the sun comes up. When it gets thick it can make a hell of a noise—especially in springtime. They’re very low tones, like thunder. (See and hear Lake Superior ice breaking up.)

What’s so addicting about Nordic skating?

It’s about finding these venues and figuring out the patterns—take this particular lake, and assess the many different dimensions.

The challenge is very satisfying. If it doesn’t work, you learn from your mistake and try again.