The search for a woman who is believed to have fallen into a sinkhole in western Pennsylvania has become a recovery effort after two treacherous days of digging through mud and rock produced no signs of life, authorities say.

The crew working to find 64-year-old Elizabeth Pollard packed up and planned to return in the morning.

Pennsylvania State Police spokesperson Trooper Steve Limani said authorities no longer believe they will find Pollard alive, but that work to find her remains continued.

“We’ve had no signs of any form of life or anything,” Limani said.

There was nothing to suggest it was worth continuing "to try and push and rush and push the envelope, to be aggressive with the potential of risking harm to other people".

He noted that oxygen levels below ground were insufficient.

Emergency crews and others have been trying to locate Pollard, 64, for two days.

Her relatives reported her missing early on Tuesday and her vehicle with her unharmed 5-year-old granddaughter inside was found about two hours later, near what is thought to be a freshly opened sinkhole above the long closed, crumbling mine.

“We feel like we failed,” Limani said of the decision to change the status of the effort from a rescue to a recovery. “It’s tough.”

Rescue workers continue their search on Wednesday. – AP

Limani praised the crews who went into the abandoned mine to help remove material in the search for Pollard in the village of Marguerite, about 40 miles (65km) east of Pittsburgh.

“They would come out of there head to toe covered in mud, exhausted. And while they were getting pulled up, the next group’s getting dropped in. And there was one after the next after the next,” Limani said.

Authorities had said earlier that the roof of the mine had collapsed in several places and was not stable.

“We did get, you know, where we wanted, where we thought that she was at. We’ve been to that spot," said Pleasant Unity Fire Chief John Bacha, the incident's operations officer.

“What happened at that point, I don’t know, maybe the slurry of mud pushed her one direction. There were several different seams of that mine, shafts that all came together where this happened at.”

Searchers were using electronic devices and cameras as surface digging continued with the use of heavy equipment, Bacha said.

In the coming days, they plan to greatly widen the surface hole, with winter weather forecast in the region.

Geological engineer Paul Santi said the chances of Pollard surviving if she slipped into the sinkhole were “pretty small”.

“There’s a lot of problems,” said Santi, a professor at the Colorado School of Mines.

“There’s rock and soil and things that could have buried her. There is water that could have filled it.

"You have to go through with the rescue. But I would be surprised if she came through this OK.

"It would require that she wasn’t killed by the fall, she wasn’t killed by the rock, that there was an air pocket and she’s able to survive in it.”

Elizabeth Pollard. – police handout

Sinkholes occur in the area because of subsidence from coal mining activity. Rescuers had been using water to break down and remove clay and dirt from the mine, which has been closed since the 1950s.

Crews lowered a pole camera with a sensitive listening device into the hole, but it detected nothing.

Another camera lowered into the hole showed what could be a shoe about 30 feet (9 meters) below the surface, Limani said. Searchers also deployed drones and thermal imaging equipment to no avail.

Pollard's family called police at about 1am Tuesday to say she had not been seen since going out about 5pm to search for Pepper, her cat. The temperature dropped well below freezing that night.

Limani said the searchers met with her family before announcing the shift from rescue to recovery.

“I think they get it,” Limani said.

Rescue workers clear equipment to take a different approach in their search. – AP

Pollard's son, Axel Hayes, described her as a happy woman who liked going out to have fun. She and her husband adopted Hayes and his twin brother when they were infants.

She used to work at Walmart but recently was not employed.

Hayes called Pollard “a great person overall, a great mother” who “never really did anybody wrong”.

He said at one point Pollard had about 10 cats.

“Every cat that she’s ever come in contact with, she has a close bond with them,” Hayes said.

The top of the sinkhole in the village of Marguerite, Pennsylvania. – AP

Police said they found Pollard's car parked behind Monday's Union Restaurant in Marguerite, about 20 feet (6 meters) from the sinkhole.

Hunters and restaurant workers in the area said they had not noticed the manhole-size opening in the hours before Pollard disappeared, leading rescuers to speculate that the sinkhole was new.

Pollard lives in a small neighbourhood across the street from where her car and granddaughter were found by state police.

The young girl "nodded off in the car and woke up," Limani said.

"Grandma never came back."

It was not clear what happened to Pepper.

What are sinkholes?

A sinkhole is an area of ground that has no natural external surface drainage and can form when the ground below the land surface can no longer support the land above, according to the US Geological Survey.

The land usually stays intact for a period of time until the underground spaces just get too big. If there is not enough support for the land above the spaces, then a sudden, dramatic collapse of the land surface can happen.

How common are sinkholes?

Sinkholes are most common in what geologists call karst terrain, which involves types of rock including limestone below the land surface that can naturally be dissolved by groundwater circulating through them. They can also happen due to old underground mines.

The most damage from sinkholes in the US tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania. Florida, for example, is highly susceptible to sinkholes because it sits above limestone.

How big are sinkholes?

Sinkholes can range in size from just a few feet wide to ones that cover a vast area spanning hundreds of acres. Their depth can also vary from just a few inches to more than 100 feet (more than 30 meters). Some are shaped like shallow bowls or saucers, whereas others have vertical walls. Some hold water and form ponds.

Other recent sinkholes

• In June, a giant sinkhole in southern Illinois swallowed the centre of a soccer field built on top of a limestone mine, taking down a large light pole and leaving a gaping chasm where squads of kids often play. No one was hurt.
   
• In 2023, a sinkhole that in 2013 fatally swallowed a man sleeping in his house in suburban Tampa, Florida, reopened for a third time, but it was behind chain-link fencing and caused no harm to people or property. Officials said the sinkhole reopening was not unusual, especially in central Florida with its porous limestone base.
   
• A large sinkhole opened up in 2020 in South Dakota near where a man was mowing his lawn. Testing revealed a large, improperly sealed mine beneath part of the housing subdivision, and a 40-foot-deep (12-meter-deep) pit mine in another corner of the neighbourhood, a lawyer for some of the area homeowners said. Since the first giant collapse, more sinkholes have appeared.
   
• A large sinkhole that swallowed oil field equipment and some vehicles in southeastern Texas in 2008 expanded in 2023 when another sinkhole developed and co-joined.