Deep in Bulgaria's wondrous karst landscapes lies a natural wonder that has captivated travelers for ages. Prohodna Cave, the country's largest tunnel-like cavern, features an unearthly pair of symmetrical holes in its lofty ceiling known as the "Eyes of God."
Perched dramatically in the mouth of a 400-foot cliff in southwest Slovenia is Predjama Castle, certified by Guinness World Records as the world's largest cave castle.