Rome's rarely seen underground archeological sites including caves where followers of the mysterious Mithraic religion used to worship are to open to the public for a week.
''Thirty sites will be open, some never visited before,'' said Rome's culture chief Umberto Croppi.
''The aim is to allow people to get to know the city's rich heritage but also to highlight the fact that these sites are always usable,'' he said.
Rome Mayor Gianni Alemanno said it was ''an experiment to repeat and extend to avoid Rome being known only for its most important monuments''.
Among the sites on show, most of which usually require a special appointment, will be two Mithraic temples, or Mithraea, under Palazzo Barberini and the Church of Santa Prisca.
These were places of worship for the followers of Mithraism, popular in the Roman Empire between the first and fouth centuries AD, and were constructed underground to resemble the cave where the god Mithras is supposed to have slain a sacred bull.
Very little written evidence of Mithraism survives, with highly secret rituals being passed from followers to new initiates.
Msgr Renato Boccardo, secretary-general of the Vatican's governing commission, said ''a piece of the Vatican'' would also go on display in the shape of the necropolis of Santa Rosa, which was discovered in 2003.
Hidden Rome: Paths in Underground Archaeology runs May 25-31.
