ASPEN, Colo. (CBS4) – It used to be an inconsistent nuisance, but now it’s turned in to one big stinking problem. And workers at one Colorado sanitation plant don’t know why it smells so bad.

There are actually several theories to the problem in Aspen, but no solution yet. The complaints from one neighborhood full of families are sporadic, but since the holidays more than a month ago they say it’s the first time the smell has lingered.

“There’s an inherent smell that goes along with waste water, there’s no doubt, no one can deny that,” said Nathan Nelson, Aspen Consolidated Sanitation District Operations Manager.

The smell that’s been coming from the Aspen waste water plant even has the sanitation district concerned.

“Honestly, I didn’t think much about it when I first moved in but they did say it didn’t smell,” Aspen resident Marie West said. “They still say it shouldn’t smell, but it smells more often than it did before.”

Neighbors usually call around Christmas and New Year’s when the town is full of people and the plant is working at capacity.

“We all just talk as neighbors like, ‘Oh, it’s really bad,’ ” West said.