Frazil ice looks like patches of snow, nestled among the trees. But it's actually a Slushee-esque mixture of ice crystals that form in bitterly cold waters—like the kind that run through Yosemite National Park in early spring. Watching this video, you can see how frazil ice can appear to be just your average slushy creek water, and, the next minute, turns into what looks like solid (if snowy) ground. And then the ground moves.
If you're thinking that it's potentially dangerous, you'd be right. In the video, park rangers talk about the risk of falling through frazil ice into frigid water deeper than your head.
(Thanks, Ferris Jabr)
Strange nature at Boing Boing: