They've been mistaken for everything from forest fires to gas well explosions -- even alien spacecraft.
But fear not, those "giant flames in the sky," as some call them, are all a part of the Marcellus Shale gas well boom here in southwestern Pennsylvania.
Looking like huge Roman candles high atop the horizon of mostly rural landscapes, gas well flares sometimes can be seen from as far as 10 miles away.
The flaming hydrocarbons spewing up a mile or more from the depths of the earth -- and sounding much like a jet engine -- can be frightening the first time someone sees it.
"For some of the bigger ones, we got calls, but once people realized it was a controlled burn, they tapered off," said Jeff Yates, Washington County's director of emergency services, about hundreds of 911 calls that the county has received in the past several years.
The flares are used at the end of the drilling and hydraulic fracturing -- or fracking -- stage, explained Tony Gaudlip, development manager and chemical engineer with Range Resources.
"What you're doing is testing the well," Mr. Gaudlip said. "You just spent a couple million dollars drilling this well, you want to see what you've got."
Flares are used to slowly release pressure in the well before the production stage, Mr. Gaudlip said.
Once a well is drilled, it expels fracking fluid and oil -- which are then stored in tanks -- and natural gas in the form of about 77 percent methane, along with secondary fuels such as propane, butane and ethane.
While a well is being flared -- which typically takes three days -- measurements are taken, along with temperature and other critical data.
When pipelines are nearby, flaring isn't necessary because the gas is pumped directly into the pipeline, Mr. Gaudlip said. The oil is sold, and the water is recycled and reused to frack other wells.
"We don't flare every well anymore," Mr. Gaudlip said. "It's kind of the exception to the rule."
Flares -- sometimes called flare "stacks" -- are typically about 20 feet high, depending on design, he said. They are very loud because of the velocity of the gas, although the sound can be mitigated with special equipment.
As with other aspects of Marcellus Shale drilling, there has been controversy over pollution, specifically air emissions from the burning flares.
Companies such as Range Resources have been flaring fewer wells in favor of new technologies, including the use of no-flare or contained-flare well completions.
Although it isn't the ideal solution, burning the natural gas is better for the environment than directly releasing methane -- a greenhouse gas -- into the atmosphere, Mr. Gaudlip said. Not flaring, which was the practice decades ago, also creates an ignition hazard.
"It's safer to have the gas burn," he said.
Dan Stevens, Westmoreland County deputy emergency management coordinator and public information officer, said his department has received dozens of phone calls during the past three years from residents who were concerned about flares.
The calls tend to peak during foggy weather or when clouds are hanging low, he said, because more light from the flares is reflected during those conditions.
As in other surrounding counties, the 911 emergency response system in Westmoreland has been updated in recent years with the GPS coordinates of each gas well and access road.
When gas wells first began dotting the landscape in the eastern suburbs, Mr. Stevens said 911 operators responded to calls about flares with a full contingent of firefighters and emergency medical personnel, but companies since then have agreed to cooperate with county officials to prevent such a waste of resources.
"Normally, the procedure is the gas well company notifies us when there will be a flare," he said.
Companies have done the same in the surrounding counties, notifying emergency responders and even local media outlets in advance of a flaring operation.
"They've been pretty good with us most of the time," Mr. Yates said. "We usually get notification."
Still, 911 operators are taught to assume nothing and to dispatch fire chiefs to investigate calls just in case, said Mr. Stevens and other emergency coordinators.
"If it's not clear and definitive, we'll send the fire department," Mr. Stevens said. "We're not going to take any chances."
Frank P. Matis, Butler County's director of emergency services (who has retired since being asked to comment,) said the state Department of Environmental Protection has been alerting his county to as many as two new drilling permits per week.
While the county's 911 center hasn't received many calls from anxious residents yet, he's bracing for them.
"I would expect us to be getting more and more, because there are more drills going in every day," Mr. Matis said.
Mr. Matis said that while his 911 dispatchers may not send out "all of the troops" to every suspected flare, it's still important to investigate.
"Our policy here is that any type of unknown fire, we're going to send the fire department out," he said.
Mr. Yates agreed, saying the procedure in Washington County, where there are far more gas wells than elsewhere in the region, is the same.
"[Calls] still get investigated, but we usually let the fire companies know that we think it's a flare," Mr. Yates said.
Although most residents of Washington County have by now seen at least one of the hundreds of flares that have burned near Marcellus Shale gas wells in the past several years, there was nothing like the unplanned flare spotted by residents near a large gas processing plant in Chartiers in December.
"We got a flurry of calls when that happened," said Mr. Yates of the flare on the night of Dec. 8 at the MarkWest Liberty facility, in which a safety control system flared off an influx of natural gas liquids. "That was pretty dramatic."
Mr. Yates said county 911 operators even got calls from as far as Allegheny County, although the safety control system worked as it should have by isolating the influx of liquids to the flare.
The gas well, and its accompanying flare, was installed last year near the Galleria at Pittsburgh Mills mall on Route 28 in Frazer.
It is one of only a handful of gas wells in Allegheny County, but it also attracted plenty of attention.
"We did have a lot of phone calls in the beginning," said Alvin Henderson, acting chief of Allegheny County emergency services. "It was something new, and people were probably concerned for obvious reasons."
Looming above the mall on a bluff along Yutes Road, the flare is occasionally lit and serves as one of the most visible ones in the region.
When concerned residents, commuters and shoppers call, it's usually to report an unknown fire or suspected incident at the well site, Mr. Henderson said.
"The calls have diminished now," he said.
Although it may seem counterintuitive, Mr. Yates said flaring and other types of controlled burns are often recommended in emergency situations, such as hazardous materials spills, to keep noxious gases from entering the atmosphere.
"That's an accepted practice in training," he said.
Although Marcellus Shale flares get more attention because of their abundance, gas companies have used the practice for years when they repair or replace gas pipelines, Mr. Yates said.
"Flares have been around forever," Mr. Yates said. "They're nothing new."
Correction/Clarification: (Published February 3, 2012) When asked to comment for a story about well flares, Frank P. Matis was Butler County's director of emergency services. Since then, he retired, which Thursday's story did not indicate.
Janice Crompton: firstname.lastname@example.org or 412-851-1867. First Published February 2, 2012 5:00 AM