This mountainside monastery, altitude 3,117 feet, was founded in the 4th century and is said to be the oldest in Greece, though it has been burned down many times, most recently in 1934. The community once had 450 monks and owned vast tracts of land in the Peloponnese, Constantinople (now Istanbul), and Macedonia, making it one of the richest in Greece. Mega Spileo sits at the base of a huge (360-foot-high) curving cliff face and incorporates a large cavern (the monastery's name means "large cave"). You can tour the monastery to see a charred black-wax-and-mastic icon of the Virgin, supposedly painted by St. Luke, found in the cave after a vision of the shepherdess Euphrosyne led some monks there in AD 362. Also on display are ornate vellum manuscripts of early gospels and the preserved heads of the founding monks. Modest dress is required; wraps are available at the entrance.
If you're taking the Kalavrita Express, 45 minutes into its trip you can alight at the stream-laced
mountain village of Zakhlorou, from where you can hike up a steep path through evergreen oak, cypress, and fir to the monastery. This hour-long trek (one-way) along a rough donkey track gives you superb views of the Vouraikos valley and distant villages on the opposite side. The occasional sound of bells, from flocks of goats grazing on the steep slopes above, is carried on the wind. It's also possible to take a cab from the village, though they are not always available; if you're driving, the monastery is just off the road between Diakofto and Kalavrita and is well marked.