Loud explosions heard in parts of Canada are being blamed on a phenomenon called frostquakes, says Tom Geoghegan.
Ontario is no stranger to temperatures approaching -20C (-4F) but in recent days the winter freeze has been accompanied by unusual sound effects - bangs waking people up.
@sydney_grieve was one of many to tweet: "Heard a loud bang outside of my room at 2am. My whole family was searching the house thinking someone broke in. #frostquake"
Emergency services in the Greater Toronto Area received reports of loud noises, prowlers and thuds on the roof. Few of those calling in expected to be told it was probably the weather.
Known to experts as cryoseisms, frostquakes happen when moisture that has seeped into the ground freezes very quickly. It expands and builds up pressure, causing the frozen soil or rock near the surface to crack, emitting a sound that people have likened to a sonic boom.
They're not very common because they require such a rapid change in temperature. In southern Ontario, a drop from 5C (41F) to about -20C (-4F) was preceded by an ice storm, which ensured there was a lot of moisture in the ground that became ice.
That doesn't happen every year, says Geoff Coulson of Environment Canada, who says the term "frostquake" is new to him. It's like a very weak earthquake and the house might shake but there is hardly ever significant damage, he says, and it will only be felt or heard within a kilometre (0.6 miles) at most.
One peculiarity is that it rarely sounds like it is coming from below, due to the way the soundwave travels - one couple thought a truck had hit their house, while others feared a tree had fallen on the roof.
"The peak time is between 3am and 8am, and that's related the depth of cold at that time," he says. But sometimes the ice slowly builds and the explosion may happen days after the temperature drops.
Frostquakes have been reported before - northern US states such as Ohio had a similar experience in 2011. But the phenomenon was not widely reported until this winter, says Coulson, largely due to social media.
Thanks for your comments about your frostquake experiences.
We have had several over the last couple of weeks, the first on Christmas Eve about 10pm. It really does sound like a sonic boom or something crashing into the roof or house. (Children on Christmas Eve thought it was reindeer coming a bit early). Then a couple of days later there were three in rapid succession early in the morning. At that point I pulled on the winter gear and went out to inspect the house. I knew there were no more branches from the big tree that could have fallen - they were all previously stripped off by the ice storm. In my mind I thought I might see some aircraft parts in the yard. Paul Gardham, Markham, Ontario, Canada
Last Thursday I heard a large boom, I thought something big fell, looked around and there was nothing, it was -28C outside. Elie Nicoghosian, Montreal, Canada
We awoke to the sound of a large boom on Christmas Eve (we told our six-year-old it was Santa on the roof), and again on January 2nd. Both were very loud, deep sounds - a bit like thunder but deeper and shaking the house to the foundation. Paul Meloche, Whitby, Ontario, Canada
I was visiting my family on Christmas Eve in Kingston (a town about two hours away from Ottawa) and we all woke up around 2am to the sound of a loud crack. Kingston recently experienced the same ice storm that affected Toronto and we were scared that maybe a tree had fallen on our garage or something. The next day we told my three-year-old nephew that Santa landed on the roof the night before and woke the whole house. He's still amazed. Sarah-Leanne, Ottawa, Canada
It happened right after I woke up on Christmas morning. There was a loud boom and some of the pictures actually fell off the wall. It was very unusual because it was just one big bang and sounded like a tree had landed on the roof. I went outside and checked but could not find any clues as to what it was. Taylor Lloyd, Gananoque, ON, CA
On Dec 31 about 9pm, I heard a loud boom and the floor seemed to shake. I thought one of the neighbours had thrown a big snowball at my window (being New Year's Eve) but when I went to the window no-one was on the road next to my house. Temp at the time was -16F. Tom Mellon, Ham Lake Minnesota
I am from Quebec, Canada and loud bangs are heard in very cold temperature, often in new or recently built houses where the cold makes the wood structure of roof and walls to contract so much that nails heads just pop-up with a loud bang, almost like a gun shot. No quake there! Luc St-Pierre, Vienna, Austria