YELLOWSTONE NATIONAL PARK, WYOMING--Bruce Fouke can't leave a rock or microbe unturned. For 10 years, the University of Illinois, Urbana-Champaign (UIUC), marine geologist has studied the origin of terraces made of travertine--a stone commonly used in floors and countertops--in Yellowstone National Park's Mammoth Hot Springs. At a meeting hosted by the Thermal Biology Institute at Montana State University (MSU) in Bozeman held here last week, Fouke presented the first evidence that heat-loving bacteria in hot pools catalyze the mineralization, a process long assumed to be largely inorganic. The finding could improve models used for gas and oil exploration and for estimating groundwater supplies.