CHICAGO — Montrose and Harding avenues looked like a scene from Yellowstone National Park for a few minutes Sunday morning.

As a torrential downpour flooded some North Side neighborhoods, an enormous spout of water shot from an overwhelmed sewer main at Montrose and Harding avenues. It wasn’t unlike the national park’s famous geysers.

Water spouts also burst out of the ground near Diversey River Bowl and at Argyle Street and Wolcott Avenue in Lincoln Square.  

Unlike Old Faithful, Chicago’s urban water geysers were no geothermal reaction. Plumbers and city officials said they were caused by an unusual convergence of water and air that created pressure at sewer junctions during the heavy rainstorm.

The city builds “deep tunnel drop-shafts” in the sewer system with vents that are supposed to release air during heavy rainfall, said Megan Vidis, a spokesperson with the water department.

But extreme rainfall, like the storm that occurred Sunday, can happen so fast that air gets trapped in the sewers. The buildup produces enormous pressure, which can be strong enough to blast off sewer manhole covers — which typically weigh about 140 pounds — and shoot water high into the air.

“Severely intense water inflows can entrap air which may produce the rare geyser effect seen last weekend,” Vidis said in a statement.

Vidis was adamant that the geysers were not caused by burst pipes. 

“Water mains do not explode,” Vidis said. 

Plumbers confirmed the issue. During heavy rainstorms, a high volume of water enters the sewer system in a short amount of time. That water, flowing downhill, often merges at a junction — or an intersection of sewer pipes — so fast that it can combine with trapped air pressure to push manholes off the ground. Manholes generally cover junctions or service stations. 

“Those manhole covers are heavy, but if you move a mass equally, it is going to lift them. And water is equally pressuring that mass,” plumber Eric Lister said.

The sewer could also be slightly disturbed from the vibrations of a car driving over the manhole, among other things, jostling them enough to create further pressure imbalances, Lister said.

And like a gasoline can, the sewer will release water very slowly with only one opening. But a second opening in the back of the can will force enough air into the container to allow gasoline to gush out — not unlike the sewer geysers, he said.

“Sometimes you need to let Mother Nature burp a little,” Lister said. 

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