It's probably the most-overlooked museo nazionale Romano — but for a history buff, or someone simply trying to wrap their head around Rome's many, many years of evolution, the Crypta Balbi deserves a stop.
The museum's big claim to fame is that it stands on remains of the Theater of Balbus (13 B.C.), and you can still go down and see the ruins, today hidden beneath the modern museum (above). While that's cool — and, after such neat underground experiences as the columbarium of Pomponio Hylas or the Mithraic temple beneath the Circus Maximus, I'm aware I might be a bit jaded biased — it wasn't, for me, the best part of the Crypta Balbi. Particularly as the signs for the underground section were rudimentary and confusing, making it near-impossible for anyone but an archaeologist to be able to figure out what was what.
So why go to the Crypta Balbi?
In all honesty, because it's the first museum I've found that lays out what the historical center of Rome looked like in ancient times, in the Middle Ages, and through to today. With accompanying artifacts.
No, it's not with cutting-edge technology. But those maps and pictures? They're pretty darn helpful. Now, when I walk past the Largo Argentina or by the Theater of Marcellus, I have a much, much clearer image in my mind of what not just particular buildings, but whole neighborhoods, would have looked like. (Below, the Crypta Balbi area in the late-antique and medieval periods).
The artifacts in the museum, meanwhile, are actually much more extensive than I'd expected, with artifacts like the Forma Urbis Romae, a 60-by-45-foot marble map of the city that Emperor Septimius Severus mounted in the Forum to help 3rd-century visitors to the city. (Today, obviously, only fragments remain. But it's still cool to see).
Despite its treasures, the Crypta Balbi isn't a particularly large museum. And that's kind of nice. It means you can easily see the underground, look at all the artifacts, and wrap your mind around ancient Rome in about an hour and a half. And, after a day at the Vatican or an afternoon at the Palazzo Massimo, don't discount the merit of not being exhausted after a museum trip.
The Crypta Balbi is open daily from 9am to 7:45pm, except Mondays. The ticket (€7 full, €3.50 reduced) is valid for three days at not only the Crypta Balbi, but also the Palazzo Massimo, Palazzo Altemps, and Baths of Diocletian. It's located at Via delle Botteghe Oscure 31. Here's a map of Crypta Balbi's location.