Undiscovered Roman inventions

Are their any yet undiscovered or unmentioned inventions within the Roman Empire that you think we will find evidence of in the future?
Or maybe we will never find those evidence but it is still possible or likely that they had the knowledge of and used certain kinds of technology?

Examples:
- Practical applications of the Heron aelopile?
- telescope?
- windmill?
- compass?
- wheelbarrow?
- others, such as...
Aelopile wasn't very practical. Others have pointed out that steam gimmicks were used.

Telescopes require good, clear glass. That's a late invention, and for quite a while it was a Venetian trade secret. Even today, good glass requires sand with extremely low impurities.

Windmill. Why DIDN'T someone come up with this earlier? ???
Wheelbarrow. Ditto

Compass. More useful (to start with) at sea, and the Romans weren't great sailors - and sailing the Med hardly requires a compass.
 
Note: The Aelophile, was an interesting steam construction by the Romans if it had been developed more, you could see steampunk Romans.
 
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