COPIAPO, Chile – COPIAPO, Chile (AP) — Nearly a half-mile below ground where 33 miners have been trapped for almost three weeks, there's no build-up of toxic or explosive gases and the air has been breathable, if hot and stuffy in their reinforced shelter. There is plenty of open tunnel for them to move around, and even water to drink, dug with a bulldozer from an underground deposit.
Mining experts say the geology and structural characteristics of the San Jose gold and copper mine in northern Chile helped the workers stay alive for 17 days before rescuers finally opened a narrow bore-hole to them Sunday. And despite the mine's history of rock falls in earthquake-prone Chile, the geology also bodes well for their survival in the weeks ahead.
The Aug. 5 collapse of the mine's main access shaft happened about 1,300 feet (400 meters) below the surface, well above where the miners were working, leaving vast stretches of passageways with air to breathe.
The gallery where the miners took refuge more than 2,200 feet (688 meters) below ground is at least 1.2 miles (2 kilometers) long, with easily enough air to last 17 days or more, said Gustavo Lagos, a professor at the Catholic University of Chile's Center for Mining.
"The caves, the tunnels, that is what saved them," agreed Wolfgang Griem, director of the geology department at the University of Atacama.
The Atacama Desert that surrounds Copiapo is considered the driest place on Earth. Some parts have not seen a single drop of rain since recordkeeping began.
Yet inside the mine, the igneous rock walls are seamed with clay that contains lots of water, which Lagos said he could see beading up and dripping down the walls behind the miners in video images from their emergency chamber.
The moist environment enabled the miners to carve water from the mine floor with a bulldozer to supplement stored industrial water. The internal runoff also means there is an essentially unlimited drinking supply, though Lagos said it could be acidic and cause minor health problems.
"It would probably contain some metals which would make it taste not so good," he said. "If it contained copper it could cause vomiting, diarrhea, stomach pains. ... But it would not kill them."
The dampness combined with warm temperatures caused by the mine's depth have created an environment so stuffy that video images showed some miners stripped to the waist to beat the heat. But they're not at risk for exposure as they would be in shallower coal mines, which can be quite chilly.
With temperatures above 90 degrees Fahrenheit (30 degrees Celsius), "we're not too worried about hypothermia," said Larry Grayson, a professor of mining engineering at Penn State University.
The trapped workers also don't have to worry about toxic gases such as carbon monoxide, or explosive gases such as methane, both common hazards in coal operations.
"Where there's coal there's methane gas, and you're going to get accumulations. Explosive gases are fairly rare in hard rock mines," said Grayson.
Copper sulfite can generate smelly sulfur dioxide during mining operations, but they've stopped that. And with copper levels in the mine's vein at less than 1 percent and gold even scarcer, the minerals "will not generate sulfur dioxide in great quantities," Lagos said.
Equally improbable is the possibility of a "rock burst," an extremely dangerous phenomenon that occurs when settling layers of earth bear down on the walls of a mine and result in a sudden, catastrophic release of stored energy. Exploding pillars can turn chunks of rock or coal into deadly missiles, and the shock wave alone can kill.
In 2007 at Crandall Canyon in Utah, a mountain "bump" — the coal mine equivalent of a rock burst — killed six workers and registered as a 3.9-magnitude earthquake, according to seismologists. Days later, a second bump killed three rescuers.
But this mine's structure is likely to be stable after the initial collapse released the stress, Grayson said. "It's probably in a state of equilibrium, at least for a while. As long as there's no new mine activity, that equilibrium would not be disturbed."
Geology also will determine how quickly the miners can be pulled up to the surface. Chile's state-owned Codelco mining company is setting up an enormous perforation drill capable of carving a hole 26 inches (66 centimeters) wide through solid rock at a rate of 15 to 30 yards (meters) a day.
The terrain at the San Jose mine is mostly diorite, an igneous rock formed from cooled magma that is considered to be only medium-hard, Lagos said.
The rescue operation's leader, Andres Sougarret, estimated it would take three to four months to get the men out.
But Davitt McAteer, a former assistant secretary of the U.S. Mine Safety and Health Administration, called that "perhaps the most conservative model."
"We ought to be able to get them out in a period of weeks, not months," he said.
Grayson predicted it could take just 25 to 30 days to reach the miners, while Lagos said it would likely be somewhere between 60 if all goes well and 120 if the effort bogs down.
The miners were lucky to be working far below where the tunnel collapsed. But the mountain itself helped provide the essentials for life.
"It was surprising for everyone in the country to find the 33 of them alive. I was very surprised," Lagos said, but "all the ingredients were there."
Associated Press writers Peter Prengaman reported this story in Copiapo and Peter Orsi reported from Mexico City.