The odd geological formation is actually a basalt volcanic plug, formed when lava hardens inside the vent of an active volcano. Thanks to its sheer geological curiosity, the site was considered sacred long before the church was built in 962 A.D. The climb to the top is well worth it: The church is a Medieval classic, its fasade dominated by multicolor stonework and Islamic-influenced tiled mosaics. While fairly plain, the interior is interesting, too, and noteworthy for its uneven flooring (due to the nature of its rocky foundation), frescoes and cave-like atmosphere. Legend has it that Joan of Arc's mother made a pilgrimage here in the early 15th century to pray for her daughter.