The oldest known human presence in Athens is the Cave of Schist, which has been dated to between the 11th and 7th millennia BC. Athens has been continuously inhabited for at least 5,000 years (3000 BC). By 1400 BC,  the settlement had become an important centre of the Mycenaean civilization, and the Acropolis was the site of a major Mycenaean fortress, whose remains can be recognised from sections of the characteristic Cyclopean walls. Unlike other Mycenaean centers, such as Mycenae and Pylos, it is not known whether Athens suffered destruction in about 1200 BC, an event often attributed to a Dorian invasion, and the Athenians always maintained that they were pure Ionians with no Dorian element. However, Athens, like many other Bronze Age settlements, went into economic decline for around 150 years afterwards.