Part of the Museo Nazionale Romana, the Crypta Balbi is often overlooked on travellers' itineraries. It's a pity: not only does it display one of Rome's more interesting recent archaeological finds, it also combines the best of the ancient with state-of-the-art technology, and is packed with displays, maps and models that explain (in English) Rome's evolution from its bellicose pre-Imperial era, to early Christian times and on through the dim Middle Ages.
The underground ruins themselves, which are open for about 15 minutes every hour, on the hour (no guide), show the foundations of the gigantic crypta, or theatre lobby, built by Cornelius Balbus, a Spaniard much in favour at the court of Augustus. The lavish theatre that Balbus built next to the crypta remains hidden under buildings but new excavations have brought to light parts of the adjacent Porticus Minucia, where grain was doled out to the have-nots in Republican Rome. New digs have also unearthed a mithraeum. Upstairs is a display of the minutiae of everyday life in ancient Rome: plates, bowls, glasses, amphorae, oil lamps and artisans' tools. The upper levels provide a fantastic view of Roman rooftops.