The Greeks visited Delphi so they could get advise from the god Apollo.
To get a prophecy
The Ancient Greeks often traveled to get advice from the Oracle at Delphi. The actual oracle herself was known as the Pythia.
They believe that because they thought they found the crack of the earth and then built the Delphi on top of it.
The Oracle (at Delphi)
Pythia oracle of Delphi An oracle is a person who tells the events in the future. Delphi is the place in Greece where the oracle was located. Ancient Greeks believed that to visit the oracle was important to learn and seek answers to their future. We still do this today, but don't call the people an oracle.
The Oracle of Delphi, I think?
She is in the building at the far end of Delphi (where the Greeks are).