Yes, it’s the Colosseum but it’s a different view than you usually see. This is the “hypogeum”, the tunnels and rooms that were below the floor of the stadium where they would keep the props, the animals, and the gladiators (and probably the slaves they set up against the lions!). There’s a photo from above and a photo from the ground level in the hypogeum. You can book a tour in the Colosseum now that will take you down there though it’s not open to the general public to just wander about. There are loose stones and areas where it isn’t safe to go.