It is the worst fear of anyone who works on a natural gas well.
A spark or an error on the job results in a potentially deadly well fire that burns out of control, causing even more danger to the experts who have to be flown in to contain the blaze.
That's the situation in Dunkard, Greene County, after something caused a Marcellus Shale gas well owned by Chevron to catch fire just before 7 a.m. Tuesday, leaving one employee with a minor injury and another worker missing and feared dead.
Raw video: Gas well fire in Greene County
This raw video from Reuters shows the gas well fire in Greene County, which continues to burn. (Reuters video; 2/11/2014)
More than 12 hours after an explosion that "sounded like a jet engine going 5 feet above your house," as one neighbor put it, the fire, fueled by the well's gas, continued to shoot flames and smoke into the air, causing a hissing sound that could be heard a quarter-mile away.
The heat from the blaze -- which caused a tanker truck on site that was full of propane gas to explode -- was so intense that first responders from local fire departments had to pull back rather than risk injury.
"They essentially retreated to let the fire burn," said John Poister, spokesman for the Pennsylvania Department of Environmental Protection, which had three people on site investigating.
State police said they were told it could take days to contain the fire.
"We're being told ... the site itself, that fire, will not be contained and we will not have access to that property for at least a few days," Trooper Stefani Plume said at a news conference Tuesday.
Experts on well fires like this were flown in Tuesday from Houston.
Patti Green, a spokeswoman for Wild Well Control, the company Chevron called in to try to contain the blaze, said it would not be unusual for a response team to let a fire burn before making an attempt to knock it down.
The question that remained unanswered Tuesday was what caused the explosion.
Though the fire was initially thought to be a "blowout" in which there was loss of control at the well head during drilling that resulted in a release of natural gas, Mr. Poister said he has been told that it was not a drilling-related accident.
Instead, he said, the well had long since been drilled and crews were on site early Tuesday morning putting in pipe that would connect the well to Chevron's gas-gathering network -- the final stage before the well goes into production.
DEP records show that Chevron's Lanco 7H well was drilled in March 2012 -- as were two other wells on the same well pad -- and had not yet begun to produce gas.
DEP's online records also show the state had not issued any violations against Chevron for any problems related to the drilling of the three wells on the well pad.
In December, Chevron was given one violation for an incident related to the well site -- for failure to comply with the terms and conditions of the state's site permit -- but no details of that violation were immediately available.
Chevron said the explosion occurred at about 6:45 a.m. Tuesday.
John Kuis, 57, of nearby Dilliner said he heard his dog Riley start growling early in the morning, seconds before he felt rumbling.
"Then the house just sort of shook and there was a big loud bang," he said.
Mr. Kuis, who lives less than a half-mile from the well, said he saw smoke and flames out of his window and at first thought his neighbor's home had blown up.
A contractor working for Chevron had 20 employees on site at the time of the explosion. Beyond the worker who was injured and the one who is missing, the other 18 workers were accounted for by 8:48 a.m., according to Rep. Pam Snyder, D-Greene.
Chevron employees came to the scene after the explosion and immediately decided to call in the experts at Wild Well Control, and police created a half-mile perimeter around the site.
No schools, homes or businesses are inside the state police perimeter, and state officials don't believe the burning natural gas is toxic, Mr. Poister said, and the fire appeared to be contained to the well pad.
Wild Well Control has an office in Southpointe, Washington County, and "prepositioned" equipment to help with well control incidents at an office in Clearfield.
But local offices are not typically staffed with advanced well-control specialists -- which Wild Well calls its "first response teams" -- who would handle a well fire or other well control incident.
Most of those level of employees in the United States are based in Houston for Wild Well, and have to be flown in, Ms. Green said. That team arrived at the Pittsburgh International Airport at about 12:45 p.m.
The team arrived at the well site, after gathering equipment, at about 5 p.m. Tuesday and began "working with us to develop plans to safely address the situation," Chevron spokesman Trip Oliver said in a statement.
Responding to such an incident is rare, even for Wild Well, one of the world's best known well-control response companies. Last year Wild Well only responded to five surface well blowouts accompanied by fires and 25 other surface blowouts that had no fires.
Molly Born: firstname.lastname@example.org or 412-263-1944. Sean D. Hamill: email@example.com or 412-263-2579. First Published February 11, 2014 9:32 AM