YouTuber Tomas N and his friend were hiking the High Tatra Mountain range along the border of Slovakia and Poland when they happened upon a frozen lake with astonishingly clear ice. The resulting visual effect – that of walking on air – is really something else. But how did this clear ice come to be?

You'll want to watch this full-screened, at the highest resolution your connection can muster (the 60 fps makes the footage especially smooth):

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I love the noise Tomas makes around the 27-second mark. What is that, exactly? Joy? Nerves? Bemusement? Is he wondering, like me, what the hell is going on with this lake?

I've spent the majority of my life living in warm climates, so my experience with frozen bodies of water is extremely limited. In fact, I'd never realized before today that lake ice this transparent was even possible. Having said that, I'm fairly confident that what we're looking at here is an example of congelation ice, which can apparently look exceptionally clear when it forms under ideal conditions – that is to say atop a still, freshwater lake almost entirely devoid of gas bubbles and other impurities like soil, sediment, or plant matter.

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University of Helsinki geophysicist Matti Lepparänta, whose research revolves around the freezing of natural water bodies (!), describes the unique optical properties of congelation ice in a chapter from The Impact of Climate Change on European Lakes (emphasis added):

Light attenuation [a reduction in the intensity of a beam of light as it masses through some medium] in clear congelation ice is very close to that in liquid lake water... Since the concentration of impurities is lower in congelation ice than in the lake water, the ice may be even more transparent than the water in turbid or humic lakes. In contrast, gas bubbles in the ice scatter light which lowers the transparency and flattens the attenuation spectrum.

There's also this, from a highly cited paper by University of Washington geophysicists Peter Mullen and Stephen Warren, titled, rather instructively, "Theory of the Optical Properties of Lake Ice." The paper's introduction sheds some light on the influence of impurities, and the rate of freezing, on the clarity of lake ice:

Following the initial freezing of a lake surface, ice is added to the bottom of the layer as heat is conducted upward through the ice. Air dissolved in the water cannot be incorporated into the ice crystal lattice; it remains in solution in the underlying water or, if the water is saturated with air, it appears as bubbles within the ice. The concentration of bubbles is larger when the freezing proceeds rapidly because the boundary layer of water is then more readily saturated with air. However, the bubble concentrations in "congelation ice" thus formed by freezing at the bottom are generally quite small, so this ice is relative clear and is often called "black ice" in contrast to the "white ice" or "snow-ice" which often subsequently forms above it.

For more information, I reached out to Stanford geoscientist Miles Traer. He confirmed that the "dream scenario" for forming a crystal clear frozen lake would involve a slow freeze, big ice crystals, close to no air bubbles, and water devoid of dust and other particle impurities.

That bit about big ice crystals is important. Ice is a solid, crystal structure of water molecules. The bigger the crystals, the clearer the ice. Traer says that, generally speaking, the slower something cools the bigger its crystals will be. For a clear example of how cooling rate can affect crystal size, look to the wide range of igneous rocks that form when the molten, volcanic magma cools and solidifies. Magma that cools quickly forms "fine-grained" rocks with very small crystals. A good example is obsidian (below, on the left), which is so smooth it's sometimes referred to as volcanic glass. Magma that cools slowly, on the other hand, gives you a coarse grain, the kind readily apparent in granite (below, on the right).

Cool, right? The effect of slow cooling on ice crystals is the same. Slow cooling = big crystals = more transparency. And guess what?

You Can Make This Stuff At Home

"When restaurants want to put on their fancy pants, they will actually use a number of techniques to remove the air bubbles that usually make ice look white and snowy," says Traer.

For many years, donning fancy pants meant buying a multi-thousand dollar rig like the Clinebell CB3030, a block-ice machine designed for turning liquid water into giant cubes of crystal clear ice. And while a lot of people still use these for producing ice in industrial quantities (maybe you're in the ice-sculpture business), people have cooked up some pretty effective DIY methods in recent years. Over at Alcademics, cocktail and spirits writer Camper English has a whole list of techniques for making clear ice at home. He's documented his trials in great detail, and goes into a fair bit of science. He even includes the step-by-step instructions to techniques that didn't work. If you appreciate meticulous methodology, you're in for a treat.

English's most successful techniques all seem to rely on gradual cooling at carefully controlled temperatures. Conveniently, they also tend to involve materials you can find around the house, like an igloo cooler, or an insulated mug. Sounds like a great weekend science experiment.