What was the location of a famous oracle consulted by the Greeks?

There were lots of oracles, but the most famous was at Delphi, where in a cave among the rocks the Pythoness (a sort of medium) would go into a trance and make prophecies. It's possible that fumes from volcanic vents (most of Greece is an earthquake zone) assisted the process. The delphic oracle was famous for prophecies that could be taken either way - as in, 'if you go to war, you will destroy a great empire' - which might, of course, turn out to be your own.