Two thousand years old and yet it's the most impressive dome I've ever seen. Yes, I've visited the one in London; so the standard was set fairly low! The fact the Pantheon in Rome is still standing and looks so fresh is a wonder in itself even before you get round to realising how nice it is in there.
It just shouldn't work. The only light comes through that hole in the roof, and that hole in the roof lets in rain. Any architect proposing the design would be deemed insane. Inside though, it's fresh, calm and beautiful.
These days it's a Catholic church and there are some altars and tombs of significance. All I could do however was look up.