Questions remain after reports of houses shaking and the sound of some kind of blast in northwest Calgary Tuesday.
"It sounded like a big metal tank or something, heavy metal, falling and hitting the ground," said John Bowes, who lives in the area.
The big bang caused quite a stir on Twitter.
"We were just in the house and we heard a large explosion," said Pat Schopff. "We thought, 'What's going on.' So we looked around the house and around the yard to see what was going on, but there was nothing there to see."
Calgary police had the HAWCS helicopter searching from high above and patrols on the ground.
"Nobody seems to know where it came from," said Sgt. Simon Watts.
"Couldn't locate or see any obvious signs of an explosion or anything catastrophic."
Enmax and local rock quarries also had no incidents to report.
But police have one theory they're investigating — a frost quake. It's where a cold snap spurs "ice booms" as water expands underground.
"It can actually give off these types of signs and I don't know for sure, but one of our dispatchers googled it for me and looked it up, which was a great help," said Watts.
Even the Calgary Fire Department sent crews all across the northwest in an investigation.