The Great Cave is a historic monastery, eight kilometers from Kalavryta and is the oldest monastery in Greece.

It was built in 362 AD. at the entrance of a natural cave in the Helmos Mountains, at an altitude of 924 meters.

Tradition wants two Thessaloniki brother monks, Symeon and Theodore to be the founders of the monastery. It is said that while they were in Jerusalem, they each saw a different perspective, with the command to go to Achaia to find the Holy Icon of the Virgin. After repeated wandering and apocalyptic dreams, they met Euphrosyne, a shepherd from the village of Zachlorou, who led them to the cave where the sacred icon was located. The image was in the cave, next to a fountain, called "The Fountain of the Daughter".
In 840 the monastery was burned by the iconoclasts. In 1285 Andronicus Palaiologos rebuilt it and has since become one of the richest monasteries. It was burned a second time in 1400 and a third time in 1600, when the church and the valuable library with rare manuscripts burned. It was burned for the fourth time in 1934 and all the relics of invaluable value were destroyed. It was rebuilt in 1936 but burned and looted by the Germans in December 1943. The Germans executed the 22 monks and all the staff of the monastery and threw their corpses from a nearby cliff. But the Holy Icon was always miraculously saved.
During the Revolution of 1821 the Monastery was a beacon of Orthodoxy and Hellenism as well as a center of resistance against the conquerors and although it received many attacks, it was never conquered.
Today, in the Museum of the Monastery, the Miraculous Sacred Icon of Our Lady of the Spider-Man, painted by Evangelist Loukas, is made of mastic and wax.
In the Museum of the Monastery are kept manuscripts of Gospels and amphibians, national costumes, manuscripts with fine miniatures, precious gold crosses with fair wood, engravings, portraits and more. In a special chapel there are many icons, relics with bones of many Saints and the founders of the monastery's founders.
In the monastery it is worth to see "Trypio Lithari", an opening through which the sunlight passes through only the two equinoxes. Even the rocky area of ​​"Kissiotis" from which the German occupation troops in 1943 threw down the monastery 's fathers as well as many pilgrims and the tower - fortress located on the top of the Great Cave.