Ancient City Underneath:
Reid's Italy has some useful information.
Le Domus Romane Di Palazzo Valentini:
A well-reviewed villa under the streets of Rome.
TicketOne has
ticket ordering information.
Church Of
San Clemente:
This church reveals layers of history from pagan temple to 15th-century
church. For an overview, see
Wikipedia.
Santi Giovanni E Paola:
A number of houses are located beneath this church. See Wiki's
Churches Of Rome.
A brief introduction is provided by
Wikipedia.
Sotteranei di San Paolo Alla
Regola: An old palace hides the remains of an ancient Roman house.
A ramp lead down well below today's street level (Tel: 06-0608).
Open by appointment only.
Catacombs Of Rome:
An introduction is provided by
Wikipedia.
The
Catacombs Of Domitilla
are recommended by different sources.
Santa Maria Della Concezione Dei Cappuccini -- Wikipedia:
Atlas Obscura
offers a good introduction. See also
Capuchin Crypt -- Wikipedia:
and
Cemetery Of The Week #15 -- The Capuchin Catacomb Of Rome
. The official web-page of the
Capuchins is
located here.
San Nicole In Carcere:
Wikipedia has
a brief description. See also
Revealed Rome.
(Via del Teatro di Marcello) Immediately south of Capitoline Hill,
it was built in the 8th century on the site of three Republican temples.
Its real interest lies in the ancient remnants it incorporates.
The church is supported by the central Temple of Juno. One can
walk down the narrow Roman street that ran between the temples.
Church Of Santa Cecilia:
See
TripAdvisor
for some comments.
Rome Sotteraneo:
A group that organizes more ambitious underground tours.