What’s that smell? Firefighters across Denver area spend weekend investigating “strange odor.”

Firefighters across the Denver area spent Saturday night into Sunday afternoon investigating reports of a natural gas smell potentially attributed to either “a weather-related phenomenon” or sewage.

The Denver Fire Department started investigating the reports of a sulfur-smelling odor on Saturday night. By Sunday morning, it tweeted, “We are unable to determine a source.” The South Metro Fire Rescue also received calls about a similar smell in Centennial and Highlands Ranch, beginning at 2:15 a.m.

Around noon on Sunday, the South Metro Fire Rescue updated that it hadn’t found line breaks. “They believe the odor may be coming from a weather related phenomenon (inversion) trapping the odor in certain areas,” according to a tweet by the Douglas County Sheriff’s Office. “Natural gas meters have pressure regulators that burp gas occasionally and that is likely what you are smelling.”

Zach Hiris at the National Weather Service Office in Boulder declined to speculate on the actual causes of the smell. But he explained temperature inversion as a frequent area phenomenon. Right now, a “really, really shallow cold air mass” spans about 1,000 to 2,000 feet high, but, “as soon as you get above that really cold layer, it warms up pretty substantially,” Hiris said in an interview.

Temperature inversions can trap odors below that warm layer. Because the Denver area has experienced around 36 hours of near-zero temperatures and relatively light winds, “my guess is that we probably continue to see or hear of reports of those types of odors continuing for the next few days because we’re going to keep those cold temperatures” until Tuesday when it begins to warm up, Hiris said.

Still, he pointed to the specific smell of natural gas being so widespread as “the first I’ve heard of that happening here.”