What makes a manhole cover go airborne during storms?

Little known fact: You don't have to go to Yellowstone to see a geyser, you can find them right here in Minneapolis. You just have to wait until it rains hard enough that water explodes out of a manhole.

OK, urban geysers are very rare. But they do happen. It's much more common to see manhole covers slightly "jumping" during heavy rainstorms.

Star Tribune reporter Andy Mannix witnessed the phenomenon while walking around downtown Minneapolis during a July storm, and tweeted a mystifying video of the hyperactive manhole covers.

But what causes it?

That's the latest question for our Curious Minnesota project, a community-driven series of articles fueled by questions from readers.

Responses to the tweet suggested possible causes: Oscar the Grouch, the clown from "It" or perhaps the Teenage Mutant Ninja Turtles having a party. Sadly, the real answer is much more scientific.

To find out more, we asked Robin Hutcheson, director of Public Works in Minneapolis. She told us that the city's storm drain system was built in the 1910s and is constantly being updated.

The downtown area sits above a network of massive storm tunnels, giant pipes that can move a huge amount of water. They're big enough for a person to walk through. However, the system can still become overwhelmed. In less urban areas of town, rain will be absorbed into yards and parks. But downtown Minneapolis is made up of impervious surfaces — aka concrete — and the water has nowhere to go but into the storm drains.

As rain rushes into the enclosed space, the pressure increases. When the pressure becomes great enough it lifts the manhole covers, causing them to "dance." Each one weighs 200 pounds.

The elusive urban geyser occurs when a smaller tunnel completely fills with water.

Minneapolis currently has 7 miles worth of storm tunnels, and Hutcheson wants to build more.

"We know that we have a capacity issue downtown," Hutcheson said.

In one July storm, seven manhole covers were displaced by the pressure.

Manhole covers have been blown off with enough force to seriously injure someone, but it usually takes a little more than rainwater. In Brooklyn, New York, a man was taken to the hospital in critical condition after a manhole cover exploded into the air and hit him on the head. However fiery, violent explosions like this are especially rare. In this case, there had been recent snowstorms, and salt from the road seeped down into cracks in electrical cables.

If you see a manhole cover that has been displaced, call 311 and the city will send someone to fix it.

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Correction: An earlier version said that no one had been known to be injured by an airborne manhole cover. In 2005, a man in south Minneapolis had three vertebrae severed and a leg broken by a flying manhole cover as he was trying to re-attach it during a storm.