The growing popularity of Greek drama gave birth to outdoor stages that could accommodate large and appreciative audiences.  The plays that have survived from the Classical period at Athens were all written for performance at one of the twice-annual festivals sacred to Dionysus before an audience consisting of the entire population of the city; but the popularity of this artistic form of expression was not confined to Athens.  To go to the theater was to take part in a religious ritual; the theaters themselves, like this one at Delphi, were regarded as sacred ground.  Plays are still performed here today at special times of the year.  The early Greeks new that sound is more audible as it travels upward. These amphitheaters took advantage of natural formations in hills or mountainsides, which were terraced and laid with semicircular rows of stone blocks.