For every meter of Rome you see above ground, there are 7 meters below. Three churches stacked on top of each other at San Clemente. Nero's buried palace beneath Trajan's baths. 600km of catacombs. A river (the Aqua Virgine) still flowing to the Trevi Fountain through a 19-BC aqueduct. Piazza Navona is the shape it is because it sits on a 1st-century stadium — the running track is UNDER the piazza.
Layer 1 (Medieval, 5-12m down): Medieval foundations, wells, cellars — visible in many restaurant basements (ask if your trattoria has a cantina). Layer 2 (Imperial Roman, 8-15m): Nero's Domus Aurea. Domitian's Stadium (under Piazza Navona — accessible at Stadio di Domiziano, €8, you walk on the original track). Caracalla underground tunnels (6km of slave corridors). Layer 3 (Republican, 12-20m): Largo Argentina temples (where Caesar was killed). The Cloaca Maxima (ancient sewer, built 600 BC — still draining into the Tiber). Layer 4 (Early Christian, 5-15m): Catacombs (600km, 750,000 graves). Layer 5 (Pagan temples): 35+ Mithraic temples under churches — San Clemente (the most accessible), Santa Prisca (frescoed Mithraeum, by appointment), San Nicola in Carcere (3 Republican temples under 1 church).
Layer 6 (Water): The Aqua Virgine aqueduct (19 BC) still carries water underground from springs 20km east to the Trevi Fountain. Vicus Caprarius (Trevi Fountain archaeological area — €4, you can see the aqueduct channel beneath the fountain). Layer 7 (Tufa bedrock): Volcanic tufa — the rock Romans carved to build everything above.
San Clemente: 3 levels, €10, near Colosseum. Domus Aurea: VR tour, €16, weekends only, book ahead. Stadio di Domiziano: Under Piazza Navona, €8. Catacombs: San Callisto or Priscilla, €10. San Nicola in Carcere: €3, 3 Republican temples. Vicus Caprarius: €4, Roman apartments + aqueduct under Trevi. Caracalla underground: €5 supplement, guided tour. Best organized: Book a "Underground Rome" tour on GYG (€30-50, 3h, covers 2-3 underground sites with expert guide).