A fire deep underground in an abandoned mine on Minnesota's Iron Range continued to smolder Saturday, but there were tentative signs that it was gradually being starved of the oxygen it needs to keep burning.

The fire in the Soudan underground mine threatens both an expensive physics research facility and a unique state park.

Laboratories operated by the University of Minnesota in the mine are worth an estimated $100 million, according to fire officials, $40 million more than was originally estimated.

On Friday, a day after the fire was detected, crews placed a cap atop the mine. They also dumped 500 gallons of fire retardant foam and 50,000 gallons of water into the mine shaft.

By Saturday afternoon, instruments showed that carbon monoxide levels were dropping underground, an indication that fire-feeding oxygen levels also were dropping. Carbon monoxide is a byproduct of combustion.

"When the carbon monoxide levels started dropping, it made people believe the cap and retardant are making a difference," said Cynthia Sage, an information officer for the Department of Natural Resources.

Firefighters haven't been able to physically descend into the mine since Thursday because of the potential danger. But they lowered a metal cage into the mine shaft Saturday morning without meeting obstructions or damaging the cage. It resurfaced covered with foam indicating the foam had reached deep into the mine. The hope is to send cameras, lights and electricity down to the site of the fire, nearly a half-mile underground, and check out timbers supporting mine tunnels. If its safe, crews can be sent back into the mine, Sage said.

"We want to get the equipment down, but not yet," Sage said Saturday afternoon.

Once crews are able to reach the mine's 12th level, they will be able to begin work on reestablishing power to the underground portion of the mine, including the sump pumps needed to keep the U's facilities dry.

The fire, which triggered an alarm at 9 p.m. Thursday, has puzzled fire officials who haven't been able to determine how it started.

Their urgency to extinguish it was spurred by the need to control it before it damages the U's facility 2,341 feet below the surface and two floors below the blaze.

The facility's equipment conducts experiments that seek to explain how the universe was formed. It operates the experiments underground to protect them from cosmic rays that shower the Earth.

No one was injured, and buildings above ground are safe.

The fire poses two threats, because it is burning timbers that line the mine shaft, and it has forced officials to shut off electric power -- and in turn, the pumps that draw out groundwater.

On Saturday night, crews planned to use about half the volume of water and foam used Friday night. The water level continues to rise, but Sage said it's impossible to know how high it has gotten.

The threat doesn't appear to be imminent. According to the university, 244,500 gallons of water not pumped out would bring levels to the door of the lab. Before the power died in the mine, sump pumps were pumping about 35,000 gallon per day, meaning the lab wouldn't start to flood until late this week.

The mine also is the heart of the Soudan Underground Mine State Park, which attracted 36,707 visitors in 2010, according to the DNR, which helps operate the lab. Families and school groups don hard hats for a dark, three-minute elevator ride that brings them a half-mile underground and shows them the area's mining past.

Firefighters rode that elevator late Thursday to estimate the damage and fight the fire with water. They haven't been back down since, Sage said.

Judging from that trip and now-broken security cameras, "property damage inside the mine is anticipated to be extensive," according to a DNR news release.

Jim Adams contributed to this report. Bob von Sternberg • 612-673-7184