Two men in Slovakia recorded themselves walking on the purest ice I’ve ever seen. While hiking the High Tatras Mountains, the men strolled across Veľké Hincovo pleso, which is frozen about 250 days of the year. The reason the ice is so clear is because of how slowly the ice congeals. The ice is actually “black ice” and you can hear it straining under them. Veľké Hincovo pleso is 174 feet deep at its deepest point.