Masaya Volcano

Geologist Ken Sims (left) and fellow climber Dennis Jackson prepare to descend into Masaya Volcano. By gathering gas samples from volcanoes worldwide, Sims is exploring how our planet is evolving and how volcanic gases cause climate changes that may even have led to the extinction of dinosaurs. Studying the gases also helps scientists understand when the volcano might next erupt and what effect gas emissions may have on human health. (Photo by Amy E. Nevala, Woods Hole Oceanographic Institution)

Scientists have special permission from Nicaraguan park officials to work along the rim and within the crater of Masaya, beyond the point where tourists venture. (Photo by Amy E. Nevala, Woods Hole Oceanographic Institution)

John Catto, 45, is a Rocky Mountain-based adventure photographer and video producer who Sims met in the early 1980s at Colorado College. This was his second trip to Masaya with Sims. (Photo by Amy E. Nevala, Woods Hole Oceanographic Institution)

French geologist Pierre Gauthier, who collaborates with Sims to gather and analyze volcanic gases, works from the rim collecting gas in a homemade device crafted from a noisy handheld vacuum cleaner powered by a motorcycle battery. ?In France, money for volcanology is tight these days,? Gauthier said. ?So we have to have a lot of ingenuity.? Gauthier, a specialist in volcanic gases, originally introduced Sims to Masaya Volcano, and he conducted pioneering research to measure time scales of magma degassing. Gauthier works for the Laboratoire Magmas et Volcans, operated by the Centre National de la Recherche Scientifique at the University of Clermont-Ferrand. (Photo by Amy E. Nevala, Woods Hole Oceanographic Institution)

Dennis Jackson, 64, is a professional guide and is the author of two books on climbing. He has been climbing and guiding with Sims for three decades. Masaya was his first trip inside an active volcano. ?This guy has dragged me to some pretty strange places,? Jackson said of Sims. ?This may be one of the strangest.? (Photo by Amy E. Nevala, Woods Hole Oceanographic Institution)

Dennis Jackson reads rules posted on a sign at Masaya's rim. One said: ?En caso de explusion de rocas puede protegerse debajo del vehiculo.? The English translation: ?In case of expulsions of rocks, you can protect yourself under the car.? (Photo by Amy E. Nevala, Woods Hole Oceanographic Institution)

British volcanologists David Pyle (right), Tamsin Mather (center), and postdoctoral investigator Melanie Witt are interested in a variety of metals emitted in the volcanic gas at Masaya. Among them is mercury, which is known to be toxic to humans. ?We are trying to understand volcanoes as a mercury source,? said Mather, a chemist from the University of Cambridge. (Photo by Amy E. Nevala, Woods Hole Oceanographic Institution)

Ken Sims (right) and Dennis Jackson spent time each day checking, maintaining, and preparing ropes and other gear for their descents into the volcano. The acidic gas can quickly degrade gear. (Photo by Amy E. Nevala, Woods Hole Oceanographic Institution)

Sims uses stackable filters attached to a gas collection device to collect extremely small quantitites of metal particles ferried to Earth's surface in volcanic gas. After the expedition, Sims brings the filters back to WHOI for anaylsis. (Photo by Amy E. Nevala, Woods Hole Oceanographic Institution)

Ken Sims begins his descent into Masaya Volcano. The trip inside takes each climber about two hours. (Photo by Amy E. Nevala, Woods Hole Oceanographic Institution)

Climbing ropes secured to rocks help ensure that Ken Sims and his two climbing partners have safe passage into and out of Masaya. (Photo by John Catto, Alpenglow Pictures)

Masaya's sloping crater floor crunched and slipped under Ken Sims' hiking boots. ?It?s like walking on ball bearings,? said John Catto. The rocks are sharp, too. ?If you slip,? he said, ?you?re going to end up looking like pizza.? (Photo by John Catto, Alpenglow Pictures)