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An interesting historical perspective on the current controversy about the cause of the Cretaceous-Tertiary (K-T) extinction has been provided by more than 2 decades of debate on an earlier but equivalent question: Are the circular, highly deformed cryptoexplosion structures, and the shock-metamorphic features they contain, astroblemes formed by meteorite impact, or are they formed by internal volcanic processes (cryptovolcanism)? During the last 25 years of this debate, numerous studies have reinforced the view that shock-metamorphic features uniquely indicate shock pressures of from about 5 GPa to > 50 GPa; no generally accepted shock-metamorphic features have been found in unquestioned volcanic structures; and no internal process has been demonstrated to produce the required shock pressures. This history therefore supports an impact origin for both cryptoexplosion structures and the K-T event. Future discoveries required to make the cryptovolcanic theory a respectable alternative include undisputed shock features, for example, shatter cones, shock lamellae, stishovite, lechatelierite, in a volcanic structure; high Ir contents (>1 ppb) in volcanic ejecta layers; and a rigorous and testable theory for producing high shock pressures by internal processes.