A giant, graffiti-filled storm sewer beneath a Wal-Mart parking lot in upstate New York may have housed a shocking setup: an underground laboratory used for making methamphetamine, police said.

Authorities in Amherst, a suburb of Buffalo, have opened an investigation into the suspected meth lab some 12 feet under the superstore parking lot, along a busy thoroughfare that leads to the town’s shops, according to local news reports.

“I was completely astounded,” Amherst Highway Superintendent Patrick Lucey told the Buffalo News. “It’s like, are you kidding me? It’s just not something you run across. I’m sure it can happen.

“Anywhere at any time somewhere where there’s the opportunity and the motive, it can happen. But this was definitely something new.”

Amherst police, assisted by state law enforcement, made the discovery beneath the parking lot Monday afternoon during a “preventative patrol,” according to CBS affiliate WIVB.

“Routine patrol, that’s what we do every day,” Amherst police Capt. Scott Chamberlin told the station.

“We check in various areas that people who might be up to no good, might be using for no good,” he added.

Crews in hazmat suits were seen climbing from a manhole, pulling out wooden pallets that, police said, were used to stand above the water flowing through a culvert that leads to a main drainage line in town.

Shoppers gathered nearby to watch investigators sort through evidence pulled from the lab, which was deep inside the storm sewage tunnel.

Chamberlin said crews fished out aerosol spray paint cans, plastic soda bottles and various chemicals — including “a liquid we believe is methamphetamine.”

But police said the contents posed no immediate safety threats, according to WIVB.

“We’ll talk to the proper authorities to figure out what we need to do to make sure that’s not accessible anymore,” Chamberlin told the station.

“We were shocked when we found out about this,” Erica Jones, a Wal-Mart spokeswoman, told The Washington Post in a statement Wednesday. “It’s upsetting and unacceptable. We’re glad the police have responded and we’ll continue to help them with their investigation. Anyone who has information that can help should reach out to police.”

Investigators plan to see whether Wal-Mart’s surveillance video shows people accessing the sewer, according to The Associated Press.

No arrests have been made.

The New York State Intelligence Center, which tracks meth labs across New York State, has released warnings about such “clandestine laboratories.”

It states:

“A clandestine laboratory is an unlawful operation consisting of laboratory equipment and chemicals that are used to illegally produce controlled substances such as methamphetamine. Chemicals found in clandestine laboratories can be hazardous. Exposure to these can damage the respiratory tract, mucous membranes, eyes and skin. Some of the chemicals can produce a fire or explosion. Immediately leave the scene and contact your local law enforcement agency if you encounter what you believe is a clandestine laboratory. Inform law enforcement if you believe children are present.”

Erie County District Attorney’s Drug Task Force, which investigates drug crimes in Amherst, says on its website that meth labs are “highly explosive and flammable threatening nearby properties” and that the chemicals used in them are “extremely toxic.”

“A meth lab can be located anywhere,” it states. “The house next door, hotels, motels, RV’s, campers, trailers, public storage facilities, the highway, forests.”

According to data from the Drug Enforcement Administration, the only other known clandestine meth lab in Amherst was found in 2009, about 4 miles from Wal-Mart.

“It’s a concern. I mean, it’s unbelievable in this area. This isn’t a bad area,” Steve Harding, an area resident, told WIVB after the recent discovery. “This isn’t the kind of thing you’d expect in a place like this — that’s the bottom line.”

Town workers were surveying the apparent crime scene Tuesday.

Lucey, the highway superintendent, told the Buffalo News that the suspected meth-makers traveled some 800 feet from the culvert’s opening to get to the apparent lab.

Police said it was tall enough for people to stand up and walk through.

“They did have a working surface. They just weren’t playing in the water, basically,” Lucey told the newspaper, referring to the pallets pulled from the drain. “They must have had their own lighting system, too, because, as I look in here now, it’s dark.”

Had authorities not discovered it, he said it could have been disastrous.

“It could have been very dangerous. It was very dangerous,” Lucey told the Buffalo News. “Forget cooking and making their lab, just the gases themselves and the environment that they were working in was a very dangerous situation.”

“Certainly, they weren’t thinking,” he added.