The Mt. Pentelikos Marble Quarry was a quarry on Mount Pentelikos in Attica, Greece. It was used by the Athenians to acquire marble for the construction of the Acropolis Sanctuary (including the Parthenon), and, during the Peloponnesian War, it was occupied by Sparta and converted into a military base and prison during the Siege of Athens. In 431 BC, the mercenary Kassandra infiltrated the camp and weakened its defenses by killing all of its soldiers and officers, and she also freed some Athenian captives.