Grotta Zinzulusa in Lecce, Italy

Accessible both by land and by sea, these limestone caverns outside of the city of Lecce extend more than 150 meters. Zinzuli in Salentine dialect refers to torn scraps of cloth, which are thought to resemble the impressive stalagmites and stalactites. A journey into these caves will take travelers through areas such as the Corridor of Wonders, the Crypt, and, most impressively, the Duomo, so named for its cathedral-like, 24-meter-high ceiling.  

Archeologists have found evidence that these caves were inhabited in Paleolithic as well as ancient Roman times. More recently, the scientists who discovered the caves in 1957—and subsequently scraped a bunch of bat guano off the insides to make them more presentable to tourists—scrawled their names on the walls.