DENVER -- They are the hidden dangers in the forests.
Tens of thousands of abandoned mine shafts are putting the lives of people and pets at risk.
The state estimates there are about 23,000 abandoned mines across Colorado.
Last winter, a 15-year-old ended up falling into one of them. The teen dropped about 100 feet when his rope snapped while he was trying to repel into a mine near Golden.
That mine shaft has since been sealed. It's one of more than 10,000 the state has closed since 1980.
It's difficult work funded by coal mines, which pay a tax to operate.
Colorado's Inactive Mine Reclamation Program is one of the most successful in the country.
About 300 mines are sealed in Colorado every year. However, it's often two steps forward and one step back.
New entrances are discovered almost as often as they're sealed.
"You'll find them all over the place. We'll go looking for mines and we'll just find new ones," said Levi Hamer, who works on a contract crew that seals abandoned mines.
Hamer estimates he works on 50 to 100 mines a year.
Mike Mussler worries about the dangers abandoned mines present. His dog Ozzie fell 40 feet into a shaft near his home in 2013.
"There were two skid marks going over the edge in fresh snow," Mussler said.
Ozzie survived, but many animals and people have died.
"Unless you're right on top of it, you probably wouldn't hear somebody calling for help," said Mussler.
Before 1980, before the Inactive Mine Reclamation Program began, the state was averaging about one death per year.
"Even if you're not killed on the initial impact, you may stay down there and starve to death," said Mussler.
However, progress is being made. The dangers are declining every year as Colorado's inactive and abandoned mines are sealed off for good.