Caves are defined as natural subterranean voids that are large enough for humans to enter. They occur in many forms, and cavernous landforms make up a significant portion of the Earth's surface. Limestone caves are the best known. Limestone, calcium carbonate, is mechanically strong yet dissolves in weakly acidic water. Thus over eons great caves can form. Caves form in other soluble rocks, such as dolomite (calcium magnesium carbonate), but they are usually not as extensive as those in limestone. Volcanic eruptions also create caves. The most common are lava tubes that are built by the roofing over and subsequent draining of molten streams of fluid basaltic lava. In addition, cave-like voids form by erosion (e.g., sea caves and talus caves) and by melting water beneath or within glaciers. Depending on their size, shape, and interconnectedness, caves develop unique environments that often support distinct ecosystems.
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