Perhaps it could be argued that the Roman Empire was simply ahead of their time, but we went from engineering marvels (of their time) such as the Roman aqueducts [1], the Pantheon [2], or the Colosseum [3], which held 50-80k spectators in 72AD, to very little of note for 100s of years following.
What I find intriguing is that the Roman Empire even 'discovered' steam power in the first century [4]. Unfortunately, they seemed to have only saw it as a toy, rather than realizing its full potential. It's fascinating to me to think about how far ahead we could be had the Roman Empire not fallen.
[1] https://en.wikipedia.org/wiki/Pont_du_Gard [2] https://en.wikipedia.org/wiki/Pantheon,_Rome [3] https://en.wikipedia.org/wiki/Colosseum [4] https://en.wikipedia.org/wiki/Aeolipile