AMADOR,COUNTY, Calif. - Ordinary thermometers and rain gauges haven't been around nearly long enough to answer all the questions being raised by global warming these days. Climate scientists have had to look for alternative instruments in the natural world.

That search has led researchers at the University of California-Davis 60 feet underground to a unique archive of California climate records: stalagmites and other cave formations found in the foothills of the Sierra Nevada.

They rise in the dark like slow-growing geological mushrooms as calcite minerals precipitate from dripping groundwater.

Visitors marvel at the spooky scenes these stalagmites and other natural sculptures, known collectively as speleothems, create at Moaning Cave, Black Chasm and other Sierra caves open for commercial tours. UC-Davis geology professor Isabel Montanez and doctoral student Jessica Oster are just as awestruck - but for different reasons.

"They have phenomenal potential for understanding rainfall and temperature patterns over periods of many thousands of years," Montanez said.

Independent climate experts also have high expectations for the cave forays. Among other things, researchers hope the cave records will help corroborate other well-preserved records of ancient climate, found in such things as ocean coral, lake sediments and bristlecone pine tree growth rings.

In their growth patterns and chemistry, speleothems capture high-resolution echoes of the surface environment, including air temperature and rainfall that has trickled into the ground. The scientists want to correlate these signals with regional climate shifts at least as far back as the last ice age, 20,000 or 30,000 years ago. In the southern Sierra, some of the climate signals may extend as far back as 2.7 million years.

It's all intended to reconstruct what's known as the paleo-climate: natural episodes of changing temperature and precipitation. By knowing more about the past, scientists may more confidently predict the long-term consequences of human-induced climate change, particularly at the regional scale, where global-climate models may not readily apply. Ultimately, better information might help policymakers plan how best to adapt to global warming.

"It can help put our current climate in a long-term perspective," said Kelly Redmond, regional climatologist at the Desert Research Institute in Reno, Nev. "It may tell us what the climate is capable of doing here - what is even possible for it to do."

Speleothem studies have been carried out all around the world but are new in the Sierra Nevada. Oster is leading the UC-Davis project as part of her doctoral dissertation.

Her first studies were based mainly on core samples of formations at two other Sierra caverns, Boyden Cave and Moaning Cave. So far, she has found traces of a cold, wet period known as the Younger Dryas, about 13,000 to 11,200 years ago, as well as a drier time about 7,000 to 7,500 years ago.

Now she's taking the research a step further, having wrangled permission to remove a sample stalagmite for more careful analysis.

Last week, Montanez and Oster put on boots and hardhats for their latest plunge into the depths of Black Chasm, near the town of Volcano, one in a string of four Sierra foothill caves operated by a company called Sierra Nevada Recreation Corp. Black Chasm was designated a national natural landmark in 1976, largely on the strength of an unusually rich array of twisty ceiling formations known as helictites.

Steven Fairchild, a self-described "cave collector" and founder of the Sierra cave corporation, led the researchers down a steep staircase to a wooden platform constructed deep beneath the Black Chasm parking lot. Visitors who pay the standard adult admission of $12.50 would turn back here at the end of a 50-minute tour.

This time, Fairchild directed his guests over a guardrail and around a thick rock wall where headlamps illuminated a slippery corner of the cave ordinarily hidden from view.

The scientists had previously scouted out a foot-high, cone-shaped stalagmite for detailed analysis. Oster plans to use a laser scanner and three-dimensional printer to construct a copy of the stalagmite, which she intends to glue back to the rock stump to maintain appearances.

The genuine stalagmite will be subjected to months of study once a cross-sectional slice has been polished and scanned into a computer.

"You can see tens of thousands of years of climate records in something like this," Oster said, "and it's about as close to a continuous record as you are going to get over this kind of time scale."