In Great Falls, Montana, an evacuation occurred when sirens went a-blaring at what appeared to be a gas leak. But then they determined that there was no gas leak. So what gives?

At first they thought it was a broken canister of mercaptan, a substance used to determine whether a gas leak is occurring. The real story turned out to be more interesting:<

Nick Bohr, general manager at Energy West, said workers at the company were cleaning out some storage areas and discarded several boxes of scratch-and-sniff cards that it sent out to customers in the past to educate them on what natural gas smells like.

“They were expired, and they were old,” Bohr said. “They threw them into the Dumpsters.”

When the cards were picked up by sanitation trucks and crushed, “It was the same as if they had scratched them.”

The chemical mercaptan is added to natural gas, which is odorless, so people can detect gas leaks. It smells like rotten eggs and is not poisonous.

All the cards combined to make a very strong smell, so as the garbage truck drove around downtown, it left behind the smell people think of as natural gas.

“It’s really, really potent,” said Jamie Jackson, a battalion chief for Great Falls Fire/Rescue.

Scratch and sniff with care, apparently…