Cave of Apollo Hypoakraios
From Project Athinai
The cave dedicated to the cult of Apollo Pythios Hypoakraios (below the heights) was located in the northwest part of the Akropolis Rock, immediately above the Klepsydra. Its walls were covered with niches holding votive plaques. Some of these plaques were offered by the nine archons of the city. Some ancient authors also called this sanctuary the Pythion.
There was a leveled terrace in front of the cave. Even during the Roman period, remnants of the Pelargikon wall still contained this terrace.
There was another sanctuary dedicated to Apollo Pythios along the Ilissos river to the southeast of Athens.
Travlos, John, Pictorial dictionary of Ancient Athens, Books that matter, New York, 1971, p. 91