The Titus Tunnel is one of the great Roman engineering works around Antakya. Started under Vespasian, continued under Titus, and finished under Antoninus Pius, it cut a long channel and tunnel through rock to divert floodwater and silt away from the harbour of Seleucia Pieria.
You can walk into the dry channel and, with a flashlight and care, continue into the tunnel. Most people do not need to go all the way through to feel the scale of the work.