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

The crew working to find 64-year-old Elizabeth Pollard packed up on Wednesday evening and planned to return Thursday morning, local time.

Pennsylvania State Police spokesperson Trooper Steve Limani said during a news conference that authorities no longer believe they would find Ms Pollard alive, but that work to find her remains continued.

Trooper Limani said there had been "no signs of any form of life or anything" to make rescuers think they should "continue 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 Ms Pollard 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 2 hours later, near what was thought to be a freshly opened sinkhole above a long-closed, crumbling mine.

"We feel like we failed," Trooper Limani said of the decision to change the status of the effort from a rescue to a recovery.

Trooper Limani praised the crews who went into the abandoned mine to help remove material in the search for Ms Pollard in the village of Marguerite, about 65 kilometres 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."

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

Ms Pollard's family called police on Tuesday morning to say she had not been seen since going out on Monday evening to search for Pepper, her cat.

The temperature dropped well below freezing that night.

Ms Pollard's son, Axel Hayes, described her as a happy woman who liked going out to have fun and at one point owned 10 cats.

The grandmother lived in a small neighbourhood across the street from where her car and granddaughter were found by state police.

It is unclear what happened to the cat.

'Lots of problems' in sinkhole

Pleasant Unity Fire Chief John Bacha, the incident's operations officer, said searchers had reached the spot they believed Ms Pollard would have fallen to.

"We did get, you know, where we wanted, where we thought that she was at," he said.

"What happened at that point, I don't know, maybe the slurry of mud pushed her in one direction," he added.

Searchers were using electronic devices and cameras as surface digging continued with the use of heavy equipment, Chief 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 Ms Pollard surviving if she slipped into the sinkhole were "pretty small."

"There's a lot of problems," said Professor Santi from the Colorado School of Mines.

"There's rock and soil and things that could have buried her, and there is water that could have filled it," he said.

He said "you have to go through with the rescue" but 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."

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 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.

Florida is highly susceptible to sinkholes because it sits above limestone.

What caused the sinkhole in Pennsylvania?

The sinkhole in the Unity Township community of Marguerite appeared above the site of the Marguerite Mine, which was last operated by the H.C. Frick Coke Company in 1952.

The Pittsburgh coal seam is about 6 meters below the surface in that area.

Underground coal mining has taken place in 29 of the 67 counties in Pennsylvania, and a wide swath of the state's south-western region is at risk for soil subsidence, where underground materials shift or fall into voids created by mining and other extraction of mineral resources.

Department of Environmental Protection spokesperson Neil Shader said the state's Bureau of Abandoned Mine Reclamation would examine the scene after the search was over to see if the sinkhole was caused by mine subsidence.

AP