It is a remarkable fact that we know much more about the highest volcano on Mars than we do about the highest volcano on earth. Olympus Mons is well known to planetary scientists, has been the principal subject of a number of papers, and has been mentioned in innumerable other papers and articles. By contrast, the highest volcano on earth, Nevado Ojos del Salado (on the frontier between Chile and Argentina at latitude 27°S) is so poorly known that there are no papers about it, and even its height (∼6800 m) is not well determined. Ojos del Salado is only one of the many volcanos in the great volcanic province of the central Andes, which extends between 14° and 29°S. The existing Catalogue of Active Volcanoes of the World (Including Solfatara Fields, Part XV, Chilean Continent, by L. Casertano, International Association of Volcanology, Naples, Italy, 1963) lists only 16 volcanos in the region as being “active,” but it is obvious after even the briefest visit that there are dozens of others that should be regarded as active. Many of these are over 6000 m high, and they include several of the earth's greatest volcanos.