Secret Cistern
In the last phase of Late Bronze Age Mycenae, the ruler of Mycenae (whoever he was) undertook the building of an underground cistern which connected the nearby Perseia spring to the citadel mount, insuring access to water during a siege. This would have been a major engineering project, involving both tunneling into the hill and laying down clay pipes. It's clear the ruler(s) of Mycenae must have feared attack, but from whom? Greek legend says that Tisamenus, the last Atreid king of Mycenae, fought against and was killed by the Herakleidai, who then took over the Peloponnese, but archaeology has shown that the Dorians did not establish their presence in southern Greece for at least another two centuries. The 'Return of the Herakleidai' is a grand-sounding story, but from the available evidence thus far it has no basis in historical fact.
So who built the cistern, and why?
Up until a few years ago, water still flowed into the cistern, which had to be closed, in fact, due to the number of accidents which occurred there. If you watch the last episode of Michael Wood's documentary series, In Search of the Trojan War, he actually tosses a pebble into the blackness, eliciting a plop as it hits water.
Nowadays, you can visit again, if you're brave enough, and bring a torch which you can rent from the entrance kiosk, but don't expect to find water at the bottom as Wood did; the cistern has been filled in. Most visitors, however, don't venture that far down. The cistern isn't an inviting place. It's cold, owing to the thick limestone plaster which seals in the damp, very dark, and somewhat claustrophobic. I would not have wanted to be one of the servant women who had to fetch water from down there in antiquity.
The building of the cistern will feature in the upcoming Orestes: The High King.