As overnight temperatures in Toronto dipped to –20 C, many people again reported hearing loud booms, which CBC meteorologist Jay Scotland says was likely the result of a "frost quake."

Many on Twitter reported hearing a loud boom overnight.

"Awaken by a loud boom — thought a family member was in trouble," said a tweet by @JanineBaijnath.

Others reported hearing similar noises overnight as Toronto and much of the eastern half of the country were under an extreme cold weather alert.

Scotland said what they heard may have been the result of a weather phenomenon called a cryoseism, often referred to as an "ice quake."

The boom is caused when water in the ground expands in extreme cold

"All of a sudden that ice starts to expand — it's like having a lid on top of a bottle, that pressure builds and builds until finally something gives, the ice expands, the pressure is released, the ground cracks and we hear what sounds or even feels like a very localized earthquake," said Scotland.

"This is not an earthquake. It's ice expanding under the ground, and it leads to a loud boom and gets folks pretty scared when it happens in the middle of the night. Very rare, very cool but very scary."

Ice quakes were also reported last week in Toronto.