The Vespasianus Titus Tunnel
A combo tunnel and canal located in Seleucia in Pieria (modern Antakya in south-eastern Turkey) begun in the first century AD and finished in the mid-second century. Built by Roman legionary soldiers and slaves (many of whom were captives from Titus’s Jewish Wars).
OMG, this is super cool. I had planned for Gaius and Allerix to travel through Turkey at some point (as Roman Asia Minor is a minor obsession of mine), but this seals the deal. Now just to find and fine-tune the plotty bits.
Here’s info on the tunnel canal from the Livius website: http://www.livius.org/place/seleucia-in-pieria/monuments/canal-and-tunnel-of-titus/
And here’s the more detailed description on the UNESCO website: http://whc.unesco.org/en/tentativelists/5903/