Roman Amphitheatre, Catania
One of the Tourism Board's guides told me that this was the third biggest amphitheatre in the Roman world in its day, after Rome and Verona. Dunno, could be, but you can't tell anymore. The place used to have three tiers, but the top two were dismantled to build other buildings, and the bottom one was mostly buried in lava and then subsequently built over. In the past Piazza Stesicoro completely covered it, but now they've excavated a part of it and you can go down inside and look around, for free, if the gate isn't locked (no ordinary schedule). There are tunnells, now gated up, that lead into other underground sections of ancient Catania. Spooky.
The strangest thing about this theatre is, that the entrance is through the front door of what looks like a small museum (not recognizable at all) in a busy traffic street. Once inside there is a ticket booth and when you walk on through the house, there is this Roman amphitheatre in its backyard! Great to stroll around, it keeps you busy for an hour or so.