Cave carvers.

Scientists have found that deep-dwelling microorganisms play a major role in carving out the caves where they live. Jenn Macalady, a cave microbiologist at Pennsylvania State University, and a team of spelunking scientists, examined communities of microbes in Italy's Frasassi caves. These microbes grow in slimy formations called "snottites" that drip from moist cave walls.

The waters trickling into the Frasassi caves create an inhospitable place for most organisms to live. That's because as oxygen-rich rainwater seeps down through limestone rocks, it mixes with hydrogen sulfide in underground water. This creates sulfuric acid, a strong acid that slowly dissolves limestone--carving out the caves.

But the slime-forming bacteria have adapted to survive in the acidic waters, says Macalady. Over time, their presence also helps hollow out caves. How? Some of these snottite microbes consume hydrogen sulfide and produce sulfuric acid as waste. This boost in acid eats away more limestone, causing the caves to grow even faster, says team member Greg Druschel.