The small town of Bagnoregio is home to one of northern Lazio's most dramatic apparitions, the Civita di Bagnoregio, aka il paese che muore (the dying town). This medieval village, accessible by footbridge only, sits atop a huge stack of slowly crumbling tufa rock in the deep-cut Valle dei Calanchi. To learn more about the area's alarming geology, visit the tiny (and free) Museo Geologico e delle Frane where information panels show how the valley floor has moved 40m since 1764.
The town is entirely given over to tourism these days and you'll find several trattorias and B&Bs dotted around its stone streets.