Cave exploration. Caver examining rare stalactites crystals of aragonite (a form of calcium carbonate) in the Bezerra cave in the Bahia region of Brazil. Conditions in deep underground caves, rarely or never visited by humans or animals, allow the formation of large mineral crystals such as these. There are a large number of karst (limestone) caves in Brazil, many extending for many kilometres and yet to be fully explored. Photographed in 2010.