Top 10 Cities for Urban Exploration

Private monasteries and churches in Rome also feature their own catacombs; this Capuchin monastery was photographed in the 1880s. Transcendental Graphics/Getty Images

Big draw: The Catacombs of Rome

You can't beat the Roman Catacombs for legendary significance: There are those who believe the Holy Grail is hidden in the tunnel beneath the Basilica of St. Lawrence.

The Vatican doesn't buy it, but that doesn't detract from the experience [source: Valsecchi]. Some of these burial tunnels, which run for hundreds of miles under Rome, date to the first century, when they were built as Jewish cemeteries; the Christian burial tunnels date to the second century and have hardly been explored, as the Vatican owns these and seldom lets anybody enter them [source: Valsecchi]. Popes and saints are among the dead buried in the Christian crypts [source: CCofR].

More to explore:

  • Manicomio della Marcigliana orphanage: Abandoned in the 1970s, now empty of everything that wasn't nailed down [source: Preuss]
  • Forum of Nerva, public gathering place dating to 97 AD, now underground and accessible via Rome's sewers [source: Capitolium].

Last, to sunny California ...