It looks like the fires might have been the product of arson. The damage is extensive. The fire spread rapidly, at one point “a mile every few minutes.”

Forest fires have burned the hillside overlooking ancient Olympia, birthplace of the Olympic Games, but stopped short of entering the site, officials say.
The museum, home to one of the greatest archaeological collections in Greece, was evacuated as the fires reached the nearby modern village of Olympia.
Fires have ravaged large parts of Greece, affecting the Peloponnese, areas around Athens and Evia island.
On Sunday five bodies were found on Evia, bringing the death toll to 56.
“The flames did not enter the site of Ancient Olympia, where all the anti-fire systems functioned,” the secretary general of the culture ministry, Christos Zahopoulos, told AFP.
A new fire protection system was installed at the Unesco World Heritage site for the 2004 Athens Olympics.
[…]
The Greek PM has implied that many fires were started deliberately.
In a nationally televised address, Costas Karamanlis said: “So many fires breaking out simultaneously in so many parts of the country cannot be a coincidence.
“The state will do everything it can to find those responsible and punish them.”
A 65-year-old man has been charged with arson and murder relating to a fire which killed six people in Areopolis, in the far south of Greece.
Two youths were also detained on suspicion of arson in the northern city of Kavala.

The death toll is currently at 56. What a terrible disaster for the people of Greece. I can’t imagine this mother’s horror as she watched the fire approach:

Dozens of charred bodies were found across fields, homes, along roads and in cars, including the remains of a mother hugging her four children.
Church bells rang out in the village of Kolyri near Ancient Olympia as panicked residents tried to gather their belongings and flee through the night, said one man who called the television station.
[…]
The worst affected region was around the town of Zaharo, where thick smoke blocked out the intense summer sun and could be seen from more than 60 miles away. The blaze broke out Friday afternoon and quickly engulfed villages, trapping dozens of people and killing at least 39. Scores of people were treated in hospitals for burns and breathing problems.