ALBUQUERQUE, N.M. -- The Big Room at Carlsbad Caverns, a room that could fill more than six football fields and reach the height of a 30-story building, inspires awe among visitors, but it's a nagging puzzle for geologists.
For decades visitors were told the cave, with its gypsum formations, was formed by the relentless drip-drip-drip of carbonic acid eating away at the limestone in a rare process that creates only 5 percent of the world's caves.
But where did all that rock go?
In most caves formed that way, subterranean streams carry off the residue. But there are no underground streams in Carlsbad Caverns.
The story scientists had been telling the tourists didn't make sense to Carol Hill, a University of New Mexico geologist.
"What really puzzled us was the size of it: The Big Room in Carlsbad Caverns is the biggest cave room in North America. That's a lot of limestone to be carried away. The room is just there, and it doesn't go anywhere," she said.
The answer was under their noses all along.
An old explanation was printed on guideposts for the 500,000 annual visitors along Carlsbad Caverns pathways: Carbonic acid was said to have seeped down to the limestone from rain runoff, slowly eating away all that rock.
But as Hill discovered, this old trickle-down theory was misleading.
It wasn't trickle-down. It was bubble-up.
And it wasn't just geology. It was biology and geology.
Diana Northup, a UNM biologist who worked with Hill, said single-cell microbes -- bacteria -- that fed on pools of petroleum under the Carlsbad region were the real cave carvers.
"The carbon compounds available in oil are eaten by the micro-organisms," Northup said, "and the product they produce is hydrogen sulfide."
This deadly gas rises through fissures until it reaches water -- and oxygen. Hydrogen sulfide reacts chemically with oxygen to produce sulfuric acid, which can dissolve whole stadiums of limestone.
This process leaves clues for geologists. Large blocks of gypsum, hard to overlook in Carlsbad Caverns and especially Lechuguilla Cave, within the park boundaries, are the chemical byproducts.
Carlsbad Caverns has drawn about 36 million people since it opened in 1923. Lechuguilla remains the province of research and is closed to the public.
Hill's book, "The Geology of Carlsbad Caverns," published in the 1980s, was fully accepted only within the past five years.
"They thought all caves were formed by underground rivers. ... The old-timers had no concept of acid coming up from below," Hill said.
In their defense, she added: "Carlsbad and the other caves in the Guadalupe Mountains are formed differently than most other caves in the world." Only about 5 percent form this way.