On Monday, temperatures at Niagara Falls dipped to -13°F.
As a result, the mighty falls have partially frozen over — with giant icicles and ice mounds covering part of the surface:
However, the falls haven't frozen over completely. Water continues to gush below the ice's surface and through gaps in the ice. And partial freezing like this is actually pretty common, occurring most winters. Most of the ice results from water that's sprayed off the falls and lands on an "ice bridge" that's gradually forming across the falls' bottom.
On a few rare occasions, however, so much ice has formed that it actually choked off the flow of water. In 1848, for instance, a blockage of ice above the falls reportedly cut it down to a few trickles. Here's a photo, believed to be from 1909, when a similar event occurred:
Recently, a pair of Canadian adventurers — Sarah Hueniken and Will Gadd — became the first people to ever climb the partially frozen falls, using picks to hoist themselves up a narrow strip of ice along the edge.
"The power of the falls is staggering," Gadd told National Geographic. "It vibrates your intestines and makes you feel very, very small. I've never experienced anything like it."
Here's what the feat looked like: