How do I book the Colosseum underground tour if I’m using the romapass for free entry?

You need to call to make a reservation for the underground tour when using the romapass. Just let the operator know that you already have entry tickets and would just like to add on the tour. They will only charge you for the tour. The romapass does not include any tours, it’s an entry pass. If using the romapass for entry you also get to visit the forum and palatine hill and have it count as the same entry since they are a combo site. Those under 18 don’t need a romapass.