The pool. Turkish-style basement spa, in the foundations of the ancient Theatre of Pompey, with hydromassage mineral-water pools.
TripAdvisor includes it among Rome's hotels with a heated pool. The Oyster guide describes a vast underground spa with mineral whirlpools, without giving their temperature.
Guide's tip. These are relaxation pools, not a pool for swimming lengths: ask about the spa's hours when you arrive.