The Colosseum and the Pantheon

My favorite 2 things so far, besides the food, have been the Colosseum and the Pantheon.
Something about these 2 massive structures that are breath taking.

The Colosseum with its brutal history and elaborate design.

Then the Pantheon... just something about the fact that we are unable, in modern times, to recreate this building (the dome is one solid slab of concrete with no rebar/steel reinforcing bars). No one knows how this was done... also there is the fact that over the course of 2000 years there is still not a crack in it.