Here's a question. What is the world's first retractable-roof stadium?
...who said the Rogers Centre in Toronto (1989)? Nope.
...Civic Arena in Pittsburgh (1961)? Good guess, and that's the conventional-wisdom answer. But there's one earlier.
Try the Roman Colosseum.
Obviously, there was no rigged-up chunk of stone and bronze hanging over the facility. There was, however, a canopy, known as a valerium, which could be draped over the Colosseum on hot days.
An artist's rendering of what this might have looked like:
Yes, there's a hole in the middle, but that really always has been the European definition of a 'roof' as far as stadiums are concerned: cover the fans and if the athletes have to play in the elements anyway, oh well. The remaining part of the top of the stadium still has some brackets and sockets used for the purpose of securing the canopy.
The Civic Center does, however, retain the title of First Retractable-Roof Stadium Built For Purposes Where People Aren't Supposed To Die. So there's that.