DENVER -- Colorado is home to more than 23,000 abandoned mines. Most once helped pump millions of dollars into the state's economy.
However, the majority have now become hidden hazards in our forests, injuring pets and people all across the state.
That's why the Colorado Division of Reclamation Mining and Safety is on a mission to seal many of them off. It's something they've been doing since 1980.
It's a daunting task. So far, over the past 36 years, only 8,000 have been sealed up.
"We've had a couple incidents where dogs have fallen into shafts. We've had some people get trapped in shafts down near Golden, so it's certainly an issue," said Erica Crosby with the Colorado Division of Reclamation Mining and Safety.
No one knows that better than Mike Mussler.
In 2013, he and his dog Ozzie were out for a walk near their Nederland home when Ozzie stumbled upon an abandoned mine and fell 40 feet down into the shaft below.
"I walked back to look for him, calling him. I couldn't hear anything and there were two skid marks going over the edge in the fresh snow," Mussler said.
Ozzie injured his back leg and had to undergo surgery, but he survived. That mine has since been sealed off to make sure no one falls into it again.
"It could have been our kids that fell in," Mussler said.
Not only is the work to close the mine entrances time consuming and dangerous, it's also expensive.
The Mine Reclamation Project receives around $2 million every year in funding. The majority comes from fees paid by coal mines in Colorado.
However, that's just a drop in the bucket.
There are at least 16,000 abandoned mines still waiting to be sealed up. Many are on private property and some land owners won't allow crews on their land to do the work.
"We have some tunnels that go back over a mile," Crosby said.
Colorado isn't the only state where this work is happening. Fifty percent of all coal taxes collected by the federal government are given back to states to address inactive mines.