The coolest aspect of the video above is not immediately apparent. A hiker shuffles carefully over ice in a lake surrounded by snow-dusted mountain peaks, under an intensely blue sky. But when the camera holder shows you the ice under his feet, it is clear.
That is, the ice is completely clear, and the cameraman appears to float above the lake’s rocky bottom.
Tomas Nunuk is the delighted cameraman. (You can hear him giggle a bit at the end.) He and his friend found the frozen lake in the High Tatras Mountains of Slovakia, reports the New York Post.
Fancy drink culture can give us a clue to this source of this alpine lake's clarity. Picture the clear ice cubes that grace the cocktails of higher end bars and restaurants, where water is purified and frozen in large blocks, custom molds or expensive machines. (Even home mixologists can’t resist taking a crack at clear ice.)
As Kevin Liu explains for Science Fare, a site written by "geeks who experiment with our food," your typical ice cubes are cloudy because they hold trapped bubbles of air from the gases dissolved in the water. The rate of freezing also has an effect—slower freezing means the crystals that form will be larger and the ice clearer. As water freezes, its density also changes, and if that happens quickly, minute cracks in the stressed ice will cloud the result.
Basically, the water needs to freeze at just below its freezing point. (Liu recommends 30 degrees F.) He writes:
The method combats all the reasons ice turns cloudy that I talked about above. The slow freezing forces gases out, creates large crystals, and allows density changes to occur with minimal stress.
There are several tricks and hacks that people use to get the gas out of water, but since the alpine lake probably didn’t have a gigantic stirrer to agitate and release gas bubbles, "high-temperature freezing" likely explains what the hikers found. And the wind was probably quiet as the lake froze, creating that smooth surface.
So a very slow freeze can give you fancy clear ice cubes or, if conditions are right, a unique experience high in the mountains.