Why there are sinkholes popping up around Minneapolis

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After hearing from concerned residents and visitors, the city of Minneapolis is addressing a growing problem around Lake Nokomis and Hiawatha: sinkholes.

According to a news release from the Park and Recreation board, sinkholes and settled grounds popping up in the area are the result of erosion. The Twin Cities had its wettest year on record in 2016, and this year has also seen above-average rainfall.

Which means the ground is super saturated. And with the soil unable to absorb any more moisture, storm water ponds are flooding nearby parkland.

On top of that, weakened areas of the city's aging storm sewer infrastructure can fail during periods of heavy rain, the release says.

"This failure causes water to seep into the ground, where it moves through the spaces and cracks between the soil toward lower elevations. The water may eventually pool, cause soil erosion and result in a sink hole," the board explains.

Officials say they're "working through a plan to systemically look at their storm water management system," but that putting in new pipes is very expensive. For now, there are plans to install more groundwater monitoring devices in the area.

The release also points out the difference between sinkholes and "settling."

"The soft, very porous, peat-like material ground that comprises much of the area slowly compresses over time, and patches of compressed soil may "settle” slightly lower than the surrounding ground," the board says. "These areas are not caused by broken or weakened stormwater pipes and are not sinkholes."

More about sinkholes

If you've ever watched video footage of a sinkhole, you know they're no joke.

Sinkholes are like holes in the ground, but much more dangerous because they can cave in suddenly, "swallowing" the earth around them. They're quite common in Florida, with its frequent rains and marshy terrain – but Minnesota?

Turns out our state has its fair share of sinkholes. 

There are 14,000 mapped sinkholes in Minnesota, Dr. E. Calvin Alexander Jr., a professor in the University of Minnesota’s Department of Geology and Geophysics told FOX 9 back in April.

More than 10,000 of them are in Fillmore County, where the city of Fountain has called itself the “Sinkhole Capital of the U.S.” for more than 25 years.

The area has so many sinkholes because of its karst topography, which is a region underlain by soluble bedrock like limestone.

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