Two hikers step onto a frozen mountain lake in Slovakia's High Tatras, and the ice is so clear that every stone on the bottom shows through, as if they were walking on air above the water. Tomas N filmed the scene and posted it to YouTube in December 2014, and the clip quickly spread around the world.
The video is a reminder of how unusual truly transparent lake ice is. Most of the frozen lakes people see in winter are white or grey, covered with snow or full of trapped air. Here the surface works like a pane of glass, and the mountains reflect in it at the same time.
The High Tatras and their mountain lakes
The High Tatras are the highest part of the Carpathians, shared between northern Slovakia and southern Poland. Their summit, Gerlachovský štít, reaches 2,655 metres, and 29 peaks rise above 2,500 metres. Glaciers carved the range into steep granite cirques, and many of those hollows now hold small glacial lakes, called plesá in Slovak.
Some of these tarns are remarkably deep for their size. Wielki Staw Polski on the Polish side reaches 79 metres, and Veľké Hincovo pleso, the deepest lake on the Slovak side, reaches 54 metres. Tatra National Park (TANAP), founded in 1948, protects the Slovak side; the Polish national park followed in 1954, and together they form a UNESCO transboundary biosphere reserve.
Tomas N described the location only as a frozen lake in the Slovak mountains, so the exact tarn in the video remains his secret. With dozens of lakes scattered across the range, the scene could belong to many of them.
Why some lake ice is crystal clear
Ice looks white when something inside it scatters light: air bubbles, snow frozen into the surface, cracks or layers that formed at different times. When a lake freezes in calm, cold, snow-free weather, the ice can grow downward slowly in a single solid sheet with very few bubbles. Light then passes straight through, and the ice takes on the colour of whatever lies beneath it, which in a shallow mountain lake means the stones on the bottom.
The same physics explains the black ice that forms on roads and the clear blocks ice sculptors prize. Snow is the enemy of clarity: a single snowfall on fresh lake ice adds a white, porous layer, and once that layer soaks with water and refreezes it becomes cloudy "snow ice". That is why these crystal-clear conditions usually last only a short window early in winter, before the first heavy snow.
Watch the High Tatras ice video
In the full clip the camera moves across the surface, and the illusion keeps changing. Close to the boots, the bottom of the lake looks within reach. Further out, the reflections of snow and sky take over and the ice becomes a mirror.
Ice thickness and visiting the Tatras in winter
Clear ice is strong ice: the US National Weather Service notes that white snow ice is only about half as strong as clear ice. Strength still depends on thickness, though, and the NWS advises staying off ice under 2 inches (5 cm) thick and treating 4 inches (10 cm) as the minimum for walking, doubling those figures for white ice. Avoid cracks, inflows and spots that have thawed and refrozen, and check with local officials before going out.
In Slovakia, higher Tatra trails, passes and peaks close each year from 1 November to 15 June to protect wildlife and visitors, but many lower routes stay open. The trail from Hrebienok to the Zbojnícka and Téryho cabins, the Studenovodské waterfalls and the winter path from Skalnaté pleso to Hrebienok are popular winter outings, and the resorts of Štrbské Pleso, Starý Smokovec and Tatranská Lomnica, reached from the town of Poprad, make good bases.
For another effect created by water over ice, explore Fox Grom's husky reflections. For a wider collection of winter textures, see patterns in frozen lakes and sea ice.
Related reading
- A Transparent Frozen Lake in Slovakia’s High Tatras
- The Sound of a Frozen Lake: Cory Williams’s Alaska Clip
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