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How does a massive sinkhole form?

And could it happen where you live?

KNOXVILLE, Tenn. — Knox County crews are pouring concrete and shot rock into a massive hole that opened up in North Knox County. They want to stop it from getting ever bigger than it's current 60 feet deep. 

Crews estimate it'll cost at least $100,000 to repair.

Which led us to wonder: How do these types of sinkholes form? 

First off, it's taken a long time for the sinkhole to look like this. 

Credit: WBIR

Dr. Robert Hatcher, a geologist at UT, says it likely started forming thousands of years ago.  

He says the rain last week could've been the last straw causing the earth to swallow Greenwell Road in North Knox County.

RELATED: 'It's a monster' | Sinkhole takes out half of a Knox Co. road

"Sinkholes form in regions where you have limestone," he said, adding that limestone eroded by ground water is to blame. 

"The material simply gets carried away in the underground drainage there." 

When the limestone is gone, the earth simply gives way. 

"Once the soil is removed from below it collapses and forms an analogue sinkhole," Hatcher said. "It doesn't have to have a trigger." 

And he says it could happen a lot of places in East Tennessee 

"A place where there is going to be a sinkhole formed--you really can't tell much," he said.