ALBUQUERQUE, N.M. – Imagine a world of green slime, red goo and dripping, bacteria-rich mineral formations known fondly as "snottites."

Sulfuric acid oozes from limestone walls. Deadly levels of hydrogen sulfide and carbon monoxide belch into the air, creating a pungent, nasty smell of rotten eggs. For good measure, throw in the fluttering sounds of vampire bats on the wing.

You are entering a little patch of underground heaven -- a subsurface step on the road to learning whether Mars is a locale for past or present life.

That’s the deep, dark view of two first-class bottom feeders -- cave-dwelling explorers Penny Boston and Diana Northup. They’re a dynamic duo hot on the trail of bacterial life amid the most hostile conditions imaginable.

But then in 1966, as a first-year college student, she tagged along on a university "outings club" trek into her first cave.

"The cave had streams in it, as well as holes," she recalled. "I tripped and fell and remember then thinking that caving was quite different compared to, say, gardening."

Northup remembered her amazement at seeing her first cave rat.

"They leave urine trails so they can go in and out of the cave in the dark," she said. "That is so cool. Imagine if you had to find your mate, your lunch and dinner, where you sleep – all with your eyes closed in this foreign environment."

Today, the 51-year-old biologist is an associate professor at the University of New Mexico in Albuquerque. A long way from her first foray underground in the mid-1960s, Northup now is working in more than a half-dozen caves, such as Lechuguilla in New Mexico. At 1,000 feet (305 meters), "Lech" is the deepest cave in the continental United States, and only 100 miles (161 kilometers) of the twisting and turning cave have been explored and mapped.

Caves offer accessible subterranean labs to study a diversity of microbial life, Northup said. These ecosystems are exposed to extreme environmental stresses and may be based on inorganic energy sources rather than sunlight.

Since 1994, Northup’s intrepid caving colleague has been biologist Penny Boston, 48, of Complex Systems Research in Boulder, Colorado. Boston's long-standing interest in Mars, and the prospect of finding life there, has driven her underground too.

"My first take on caving was raw. I went in with little training on an 11-person team. It was a five-day trip where you carried everything in on your back. We were so exhausted. I thought I was going to die," Boston said.

"My first take on caving was raw. I went in with little training on an 11-person team. It was a five-day trip where you carried everything in on your back. We were so exhausted. I thought I was going to die."
     

When not looking for life in all the right places, Boston is busy doing blazing flamenco steps in a Boulder dance troupe. She also enjoys fine art, lace making and likes to read science fiction and poetry in her spare time.

Northup, a biology graduate student, has little in the way of free time. Most of her time is spent as a key librarian at the university’s Centennial Science and Engineering Library. On weekends, she goes caving for fun.

Boston said that Mars exploration and her caving adventures are a perfect fit.

"We’re looking at things that allow bacteria to function independently of the surface. It brings up the question of whether or not you can have a planet where the entire biosphere is restricted to the subsurface because the surface is uninhabitable," she said.

With no water, a peroxide-rich soil and a surface saturated with ultraviolet and cosmic radiation, Mars today is a hostile place for surface life.

Yet, for life on the surface, Earth may once have been a hostile place too, Boston said. There is a growing interest among scientists that life on Earth may have existed underground long before moving topside.

"If that’s true, why not on Mars too?" she said.

Caves on Mars? "Yes, they are there. Where you have flowing lava, you have the possibility of forming tubes, bubbles and other landforms -- like caves," Boston said.

It is clear that Mars once sported a much wetter, warmer and more hospitable climate. Here on Earth, a drying out period leads to species retreating to caves.

"Life could have developed on Mars. Then, as the atmosphere went away, maybe it found refuge in these hypothesized caves," Northup said.

Boston and Northup are busy eyeing numerous cave bacteria they have snagged from their underground sojourns. The researchers are also looking at how microbes influence corrosion in caves.

"They actually alter their environment and leave chemical signatures. By knowing more about how bacteria operate in cave environments, we can perhaps gather clues on how life on Mars might have survived and adapted," Northup said.

Plowing through collections of electron-microscope images, they ooh and ah at their captured denizens of the deep. "phlegm balls," "punk rock," "hairy sausages," "beads on a string" and "snottites" are some of the more colorful names they have coined for their novel microbe menagerie.

Caving is no walk in the park.

You can find yourself crawling through long, tight passages; struggling to pull yourself up or lowering yourself down on a rope; or even stumbling into a fissure after lighting equipment fails.

Both Northup and Boston have the bruises, broken bones, twisted ankles and pulled muscles to attest to the danger.

Despite the injuries, Northup said: "When we go into the caves, it’s a good time."

But she admits to having one bit of paranoia. "I’m deathly afraid of heights. I die a few thousand deaths when I’m being pulled up. That’s why Penny sings to me on the way up. She sings such uplifting songs," she said.

Thanks to their caving experiences and biological research work, both scientists are certain that life elsewhere is a sure find. There is no doubt in their collective minds.

Is there life on Mars?

For these cave women, being in the dark is throwing some light on the answer.