Underground sites in Rome

My wife and I visited the Catacombs of San Callisto on our 2001 trip and thought it was very worthwhile, even though the guided tour was limited. It used to be that on Sunday they closed off the Appian Way to auto traffic on Sunday, which strangely evoked the times in which it was built. Or at least that's the way it felt to me at the time.

The Domus Aurea of Nero is open sporadically, usually weekend when restorers are not working. You can check out open date at Coop Culture. Coop Culture is a site worth exploring for other archaeological sites in Roma and elsewhere in Italy.

Closer in to the city are the Catacombs of Priscilla off Via Nomentana (easily reached by bus).

The only underground sites we've been able to visit (so far) without a guide/tour have been some churches with crypts or Roman ruins under the foundations: the Vatican grottoes (not to be confused with the Necropolis), San Clemente and San Nicola in Carcere come to mind. The Capuchin 'bone' crypt at Santa Maria della Concezione dei Cappuccini may also be visited on one's own.

We did the catacombs of San Sebastiano out on the Appia Antica, and tours were mandatory there as I believe they are at any of the catacombs which are open to visitors (tours provided at the sites).

There are not too many underground sites that you can visit on your own. Most require you to be on a tour. Some that come to mind are San Clemente and the Vatican grottos.

The catacombs all conduct guided tours, they cannot be seen on your own. Each catacomb has their own guides so just show up and join the next group.

My favorite underground tours are the underground/third tier of the colosseum and the Palazzo Valentini. You can book them online at:

http://www.coopculture.it/en/colosseo-e-shop.cfm

Don't forget to get entry ticket tickets as the tour tickets are just for the tour and not entry.

http://www.palazzovalentini.it/domus-romane/

This tour ticket includes entry.

Some other great underground tours include the Via Triumphalis tour at the Vatican and Neros Golden House, though that is only open sporadically.

http://biglietteriamusei.vatican.va/musei/tickets/do?action=booking&codiceTipoVisita=165&step=2

Donna

I would echo the recommendation to visit Basilica San Clemente. Here is their website. It's a short walk from the Colosseum and does not require a tour guide. You just pay and go in.
There are multiple layers of places of worship, the lowest being a pagan temple from the 1st century. Even the church at the surface is below street level. I think it's one of the best places in Rome to get a sense of why a subway is so hard to build there! It's truly astonishing seeing how long humans have inhabited this place and the evidence they left behind.

Would second Donna's suggestion of the Palazzo Valentino. The tour is done by the site's guides and numbers are kept quite low. It is a clever and somewhat different experience as special effects/lighting are used to 'recreate' features.

The Scavi tour that goes below St Peter's is well worth considering. We booked ahead and there were about twelve in our group led by a young priest--an amazing place.

Just want to say some of the links posted on this thread are awesome! Bookmarking away...
Cynthia

Thank you so much for all the wonderful suggestions. The only underground site I've visited previously is San Clemente and I really enjoyed it. I'm looking forward to visiting some more of the sites suggested above. I only wish I had the time to visit them all!