Four kilometres east of Civitavecchia, in open countryside, the Terme Taurine are one of the largest and best-preserved Roman thermal bath complexes in central Italy. The first baths were built in the late Republican period, then dramatically expanded by Trajan in the early 2nd century — the emperor had a villa nearby — and remained in use until the 5th century. What survives is impressive: long stretches of brick walls standing several metres high, the floor outlines of the caldarium, tepidarium and frigidarium, fragments of mosaic flooring, and a dramatic semicircular niche that probably held a cult statue.
The site is open most days except Mondays, with modest admission. A circuit takes about 90 minutes. There is no shade and very little signage in English; bring water and a hat. Reachable by taxi from Civitavecchia (about €15 each way) or a half-hour walk uphill from the eastern edge of town. If you only have time for one Civitavecchia sight, this should be it.