Video shows hikers 'walking on water' Real or hoax?

A viral video shows hikers moving across a frozen mountain lake in Slovakia, where the water looks too clear to be real.

A video of hikers walking across a frozen lake has admirers shocked at the surreal footage and naysayers calling the filmmaker's bluff.

The video shows Tomas Nunuk, 38, of Bratislava, and a friend walking on ice as clear as glass in the High Tatras Mountains in Slovakia.

From across, it looks like they're walking on water.

From above, it looks like they're walking on air.

But one commenter on YouTube, where the video has nearly 700,000 views, said he had tried something similar and the ice collapsed under his feet.

"I'm calling fake on this one guys," user Narwhal Bacon wrote.

Others called photoshop bluff.

But another commenter said he'd been to this mountain lake.

"Saw this in person actually, wasn't brave enough to walk on it, but the altitude of this lake helps (somehow) with the clarity, is what the woman near there told me," wrote Keegan Austin.

Another commenter suggested it had to do with recent weather conditions that make for perfectly clear ice. Indeed, high temperature freezing – when water at a mild temperature freezes slowly to just below its freezing point – makes for clear ice.

Ice clarity has much to do with purity and the rate of freezing. Water that is cooled slowly has time to reject impurities (gases, organic material, minerals) that disrupt the careful crystal lattice that forms as it freezes. (Check out how to make really clear ice at home)

It also appears that the layer of clear ice as relatively thin and not frozen to the bottom of the lake, given how carefully the hikers are navigating the lake. 

Whatever the case, people from around the world are fascinated by the frozen phenomenon.

"I've never seen ice that clear in my life," wrote Shawn G. "I'm Canadian so I've seen quite a bit of ice."