Anders carefully guides his runners along the edge of the ice.
A mere step separates him from the pitch-black water.
With his walking stick he strongly hacks into the ice beneath his feet. Nearby tourists hold their breath.
They are watching their ice-tour guide from a safe distance but he knows what he's doing. He knows this ice, stretching amid the archipelago of islands surrounding Sweden's capital Stockholm.
He lives in his ice skates during the winter, always searching for the perfect ice.
"Those who want to find it must look at the water," Anders advises.
The Swedes have a name for people like Anders: "isbitnad" - bitten by ice.
With his bare hands the 60-year-old feels into a hole that he has carved out with the tip of his stick. The Baltic Sea beneath him is 30 metres deep.
Anders' measurement of the sparkling layer of ice on the sea comes to 10 centimetres, thick enough to bear the weight of a group of skaters. The layer has to be at least seven centimetres, he explains.
Usually this is not a problem in Sweden. In the cold winter months of January, February and March, the ice of the frozen lakes and bays measures up to one-metre thick. To bear the weight of a truck, the layer must be 25cm.
The ice is calling.
Expertly pushing away, Anders glides off - left, right, left, right - in smooth and steady steps the Swede appears to be sliding effortlessly over the clear surface.
But it isn't as easy as it looks. For those who only rarely get out on ice skates, an ice-skating tour is a real challenge.
The runners are actually long, thin blades honed to razor sharpness. They are almost half-a-metre long, but only a few millimetres thick, attached by bindings to normal winter boots or cross-country skiing boots. Similar to cross-country skis, the heel is usually free.
The first steps out on the ice are a very shaky matter. Shoving, scraping and stumbling follow for a while until one finds one's rhythm and starts to glide over the ice.
Anders expertly leads the way. The pace is steady. There's a clacking sound when the ice skate strikes the ice and a scraping sound when it glides. Otherwise, the only sounds are the howling of the wind and one's own heartbeat, amid a pristine landscape.
Occasionally, the vague shapes of other skaters appear on the horizon. Skating one behind the other in long lines, they are bathed in a glaring sunlight that reflects off the ice. If the wind is pushing from behind, you keep going, onward and onward.
Around 300,000 skaters are currently organised among 30,000 clubs in Sweden, with membership rising.
Fans of long distance, or Nordic, skating on natural ice, which was invented by the military about 100 years ago, are also to be found in Germany, Sweden and Italy.
There's no stopping the advanced skaters on the vast stretches of ice. Bent deep at the knees, their torsos leaning forward, they can reach speeds of up to 30km per hour. In a single day they will cover more than 60km. Anyone trying to keep up will quickly break into a sweat, clothed as they are in long underwear and wearing fleece jumpers beneath their parkas.
Anders, the guide, is able to hear the danger. Natural ice is never uniformly thick.
If there is a deep, muted sound beneath the blades, then the ice is stable. But the higher the scraping sounds, the thinner the ice. If there is a cracking sound, alarm bells go off.
"Humans have a sensitive ear when it comes to breaking ice," the guide notes, believing it is a survival mechanism dating back through the ages.
With a grin, Anders assures us that nobody on his tours has ever drowned. Anyone breaking through the ice is more likely to freeze to death. The water beneath the ice is around zero degrees, so every minute of exposure counts.
The guide never permits anyone on the ice without two ice picks.
These instruments are hung around each skater's neck, as well as a whistle. A skater can use the ice picks to grasp onto the ice and claw himself back to safety.
The backpack, too, which every skater is required to wear, can save one's life. At nine kilos it is not an easy load but inside there is a water-tight sack with a change of dry clothes. Anyone breaking through the ice and getting wet must quickly change into the dry clothing. Besides carrying the reserve clothes, the backpack can also serve as a swimming vest.
"It brings the person who has broken through back up to the surface," Anders explains.
Rescue ropes and hooks are also attached to the backpack.
Meanwhile, among the countless islands surrounding Stockholm, the skaters, whether sprinters or those who go at an easier pace, meet for a noon-time meal. And after 20km of skating, even a simple grilled sausage tastes like a banquet dinner.
Further information: For more on Stockholm see visitstockholm.com.