Saint Michael's Cave is a series, or network of caves made of limestone, which are found on the Rock of Gibraltar. St Michaels Cave is located on what is called the Upper Rock, inside the Upper Rock Nature Reserve of Gibraltar.
The cave was created by the slow seepage of rainwater through the rock, which turned into a carbonic acid solution that actually dissolved the rocks of the cave. The process made the tiny cracks of the geological faults of Gibraltar grow into very long passages and deep caverns over the thousands of years of its formation. The Cathedral Cave, part of St Michaels cave was at one time thought to be bottomless, and was long spoken of in the legends of Gibraltar.
The Rock of Gibraltar has long been considered to be one of the pillars of Hercules, and this too adds to the mystique and legend, and since it hosted the cave, the caverns themselves were thought to be the Gates to Hades, or Hell, an entryway to the Underworld where the dead rested.
In the latter part of 1974, proof that the cave was known to and used by prehistoric men was made clear with the finding of art on the cave walls, showing an ibex drawn there that was traced to the Solutrean period (dating the cave art to about 15-20 thousand years ago), but later, two Neanderthal skulls that were found in Gibraltar tell us that this cave could have been discovered and used as early as 40,000 BC.
References:The ancient Argos Theater was built in 320 BC. and is located in Argos, Greece against Larissa Hill. Nearby from this site is Agora, Roman Odeon, and the Baths of Argos. The theater is one of the largest architectural developments in Greece and was renovated in ca 120 AD.
The Hellenistic theater at Argos is cut into the hillside of the Larisa, with 90 steps up a steep incline, forming a narrow rectilinear cavea. Among the largest theaters in Greece, it held about 20,000 spectators and is divided by two landings into three horizontal sections. Staircases further divide the cavea into four cunei, corresponding to the tribes of Argos A high wall was erected to prevent unauthorized access into the theatron and may have helped the acoustics, but it is said the sound quality is still very good today.
Around 120 CE, both theaters were renovated in the Roman style.