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St. Paul Fire Department officials are once again urging people to stay away from openings that lead into a complex cave system by the Mississippi River after crews were called to rescue two teenagers Friday afternoon.

The teens, who entered a hole the city had already planned to close Saturday, emerged by themselves and without injury, according to St. Paul Assistant Fire Chief Mike Gaede.

The man-made system, located near Wabasha Street and Plato Boulevard along the southern bank of the Mississippi, has posed a danger for curious teens and underprepared explorers for decades. It has claimed the lives of a handful of people since the early 1990s and led to countless rescue operations by the fire department.

"These caves are not safe, they're illegal to be in … and they could become dangerous very quickly," Gaede said.

The city has found hundreds of holes leading into the caves, most of which are dug by trespassers. They can range from 10 feet to more than a mile in depth and can connect to other underground tunnels.

"They are very extensive, very elaborate," Gaede said.

Whenever a new opening is found, the Fire Department's technical rescue team is called to clear out the hole. The city's parks and recreation services then hires crews to permanently seal the entry.

Still, groups, communicating both off- and online, share which holes have been closed and which entryways remain open, Gaede said.

"We're constantly trying to stay up with them," said Gaede, who was previously part of the technical rescue team and is now the chief of fire and rescue operations.

And with good reason. People can and have become lost, injured, or killed while roaming inside the cave system.

It is a "zero-light situation" underground, he said. While people use their cellphones to light the way, those batteries can quickly be depleted.

"It becomes a matter of life and death, because that's the only way to see where you're going," he said.

To make matters worse, he added, there is no cellular service, making it impossible to call for help from below.

People can lose their way inside the tunnels or trip and fall and be injured, Gaede said. There are also high levels of carbon monoxide, which is odorless and undetectable, leading people to become lightheaded, lose consciousness or die from poisoning.

In 2004, three teenagers died from carbon-monoxide poisoning after entering the caves. Two 17-year-old girls were found dead in the caves 12 years earlier.

When called to respond, the technical rescue team brings all sorts of gear and safety equipment, including ropes, lights, shovels, GPS systems and air-monitoring equipment that can track the quality of the air inside the tunnels, Gaede said.

Earlier this week, the city had identified an entry and had reached out to the fire department with hopes of clearing and closing the hole Saturday, Gaede said.

On Friday afternoon, nearby business owners saw the two teens enter the hole. When they didn't come out after about 40 minutes, they called 911, Gaede said.

The technical rescue team arrived and found the teens. They were not injured and walked out by themselves.

The Fire Department finished plugging the hole Saturday morning, Gaede said. Crews will permanently seal it on Monday.

Miguel Otárola • 612-673-4753