Roman Concrete

[Lymania]

Relating to Sonja's comment in Brin Digest 1726

I don't think we lost the formula for concrete after the Romans had it.
But I am SURE that we lost and then rediscovered the Roman formula for 
concrete that will harden underwater.

Concrete is a pretty good contender for greatest invention.
There's a case for ovens though.  From ovens you get ceramics, bricks that 
endure, and best of all: bread.
Dan