Perhaps you thought you heard a rumble, a reverberating ping, a sharp crack like a pistol shot or an eerie zinging that moved underfoot. Have the flying saucers arrived, or is Mother Nature playing tricks on your ears?
Kenai Peninsula residents are not imagining odd noises outside these days.
The source is the area lakes as they freeze. The weather, the season and the peculiar physics of water are contributing to a rare and weird natural concert.
"It is very cool. It's fascinating," said Dave Schmidt, the physics teacher at Skyview High School.
An outdoorsman who has enjoyed many years of ice fishing and other peninsula winter recreations, he has heard a lot of strange lake noises.
"Some lakes just have that rich sound, almost like a whale," he said.
The basics of the ice noises are straightforward.
"It has to do somehow with the movement of the ice," said Robert Ruffner, a geologist and adjunct physics instructor at Kenai Peninsula College.
"Water does this weird thing. It's densest at 4 degrees Centigrade."
Water, the substance that dominates our planet, is unusual in that its cold, crystal form takes up more space than its liquid phase. If it were a "normal" molecule, lakes would freeze from the bottom up and ice cubes would sink in a drink.
Ruffner described how an entire lake has to cool to that 4 degrees before freezing can begin. Until that point, the cold water sinks and warm water comes to the surface. When all the water chills past that crucial point, the movement reverses and the coldest water comes to surface, gives up heat and, at the freezing temperature of 0 Centigrade or 32 Fahrenheit, forms the ice surface.
But as the ice thickens, it pushes up from beneath and stresses the solid surface.
The lake sounds appear to be associated with rapid drops in temperature, thickening ice and cracks.
Sometimes the weight of a person walking across the surface can trigger cracking. That can be a disconcerting experience.
"I was on a lake the other night, and one formed right under my feet," Schmidt said. "It is eerie."
But the details of the unnatural-sounding natural sounds remain mysterious.
"I am not aware of any research that has been done," said Martin Jeffries, a research professor of geophysics at the Geophysical Institute at the University of Alaska Fairbanks who studies lake ice in the Interior.
"It's sort of like an ice quake," he said.
The snapping sounds are sudden cracks, and the pinging may be from slow cracks, he said. His team also has heard deep thunking noises, which may be from ice falling into subsurface air pockets.
As the temperature falls, ice thickens vertically, but at the same time it contracts horizontally at the surface. The effect is most pronounced when the ice is fairly thin, the temperature plunges and insulating snow is missing.
"You have got this thermal stress on the ice," he said.
Cracks may start the vibration, but the physical characteristics of the lake and lay of the land around it shape the sound that reaches our ears.
"It is, I suppose, rather like a drum," Jeffries said.
Ruffner speculated that those characteristics create the resonance and reverberation that make the frozen lakes sound so queer. The pitch, volume and duration would vary with size, depth, bottom shape and how thick the ice layer is.
"Those sound waves are bouncing between the lake and ice," he said. "There is probably some optimal depth."
Jeffries noted that the Kenai Peninsula, with its maritime climate, may be particularly prone to the temperature swings that prime ice for popping.
The phenomenon seems to have escaped scientific scrutiny and could be an interesting study topic, he added.
"It is an interesting thought," he said. "You could do some interesting curiosity-driven research."
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