VIDEO: The exact moment yesterday when Saultites said ‘what was THAT?’

At 1:45 p.m. on Monday, Veikko Alopaeus had risen from his afternoon nap and was talking to his sister on the phone.

It was then that he heard the boom that had Saultites talking for the rest of the afternoon.

Like many of us, Alopaeus is used to hearing sounds from the steel mill.

But this time, it seemed different. This sounded louder.

"This was very loud indeed," he told SooToday.

"I was inside the house and I jumped. I said: 'Something exploded!'"

Alopaeus's wife and sister both figured it was probably thunder from a passing storm.

He disagreed.

"Thunder rolls," he insisted. "That was an explosion."

Alopaeus captured the big boom on his security camera.

He posted the video on social media without knowing what the sound was.

"The video doesn't sound as loud as it sounded in my ears."

It was only later that he learned he had captured the boom from molten slag encountering water from yesterday's heavy rain.

The resulting reaction caused a fire in a piece of mobile equipment at the plant, causing one employee to be treated on-site for undisclosed injuries.

The noise was not only heard by residents in virtually every part of Sault Ste. Marie, but also by people living upwards of 30 minutes outside city limits. 

Several locals living near Algoma Steel compared the sound of Monday afternoon’s incident to the one that occurred last week.

But this time, social media was set ablaze as dozens of comments flowed in on local Facebook group from Gros Cap, Carpin Beach and the Sault Airport.

Others heard the boom in even further areas like Heyden and Goulais.

“I hear bangs from the slag dump all the time, but nothing like that,” one man wrote. “It actually set me back a bit. Whatever it was, I hope nobody was hurt. That's the loudest thing I've heard. And I've been living here for fifteen years.”

“It sounded like it was right outside my door, the house shook a bit,” a local woman said.

“Manitou Park, our whole house shook. Thought a tree fell down on our house,” another resident commented.

Before Algoma Steel confirmed it was another case of water coming into contact with slag, readers had a number of theories on what the mystery sound could have been.

A small quake, a meteor, and a sonic boom were among unique guesses.

To rule out any extraordinary theories, SooToday reached out to the Canadian Space Agency to confirm whether something may have entered the atmosphere.

Their reply stated that the “CSA does not classify unidentified sounds which may originate from space, or unidentified aerial phenomena (UAPs).”

The Sault Ste. Marie Police Service said it received a couple of phone calls from members of the public concerned with the sound.

Spokesperson Lincoln Louttit said police were not aware of what could have caused the big boom.

- with files from Alex Flood