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.
Black ice is recently frozen, and can be as thin as 2 inches and still support the weight of a skater. Like a dome or arch, the support comes from the sides rather than the top, and the water underneath prevents the ice from breaking. But, experience and careful advance planning are key. Factors including temperature, atmospheric conditions, and even satellite images of the Earth’s surface are considered. It’s complicated, but appealing to mathematicians like Ajne. Even with preparation, there’s a risk of falling in… So it’s best done in groups, for safety.