Caves have human and economic value for recreation, mining, and other activities. For scientists, however, caves are valuable because they offer a unique habitat for rare forms of life. The study of these cave-dwellers has contributed to our knowledge of biological adaptation and evolution. In addition, medical researchers have cultured hundreds of cave-dwelling organisms to test their ability to combat diseases, including cancer and malaria.

In Wyoming's Kane Cave, scientists have begun to unravel a mystery that may have consequences in the debate about whether life can exist on other planets. Certain microbes that inhabit the inner recesses of this cave system survive in conditions that humans and most other organisms would find inhospitable, even deadly. These extremophiles — organisms that live in conditions outside a normal range — live mostly off simple inorganic chemical compounds such as iron and sulfur.

Some extremophiles thrive where it is hot, others where it is dark, and still others where pressure is extremely high. Some, it turns out, need all three conditions. Bacteria found deep in the volcanic regions of mid-ocean ridges suggest a possible scenario for the beginning of life on Earth. Water in deep-sea hydrothermal vents reaches temperatures of 375° C (707° F) and higher, and is rich in dissolved minerals. When hot water rising from the vents in geysers meets the cold ocean water, it cools rapidly, and minerals precipitate out of solution. In some places, the resultant compounds of iron and sulfides form "chimneys" on top of the vents. Microbes survive in this environment using only these simple inorganic chemicals for energy. Similar primitive conditions existed on early Earth.

A parallel may also be drawn between the microbes of the deep-sea vents and those in Kane Cave. Even though the microscopic inhabitants of Kane Cave prefer acidic conditions — in which they feed off hydrogen sulfide in the air and produce sulfuric acid — like those in the deep-sea vents, they survive using only simple inorganic chemicals.

Finding life in unexpected places and in conditions that are inhospitable to humans gives scientists reason to believe that life could also exist in other unexplored regions of Earth — including miles beneath the surface — and on other planets as well.

Create a microorganism-rich mud column in this NOVA classroom activity.

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