After years of restoration work, the Priscilla Catacombs in Rome were reopened last fall. This 8-mile network of tunnels was used for Christian burial and worship from the 2nd to 5th centuries. The catacombs were forgotten, looted, ignored, and ultimately reclaimed, and now centuries of grime has been removed to reveal striking art work.
The tombs sprung from the burial of Manius Acilius Glabrio, executed in the 90s for his faith. His wife, Priscilla, had him buried in a former quarry, and then gave the property to the Church for other burials. Numerous popes and martyrs were entombed there.
If you can’t travel there in person, you can poke around inside a portion of tunnels on GoogleMaps. It looks good, and is a nifty way to get a sense of the place.
The Catacombs have a very good website (albeit in Italian) complete with videos of the restoration.