The drowned harbour Herod built for Augustus, enormous hydraulic-concrete blocks poured underwater two thousand years ago, the…The drowned harbour Herod built for Augustus, enormous hydraulic-concrete blocks poured underwater two thousand years ago, the breakwater foundations, fallen columns, anchors and amphorae, all in a few metres of water on a marked trail you can swim with a mask. Sebastos was the largest artificial harbour of the ancient world, built by Herod between 22 and 15 BC using volcanic ash shipped from Italy to make concrete that sets underwater, Roman engineering at its most ambitious. It began to subside within a century, probably on unstable seabed and possibly after a tsunami, and the sea took it back. Anchorage: No anchorage in the ancient harbour itself; Herzliya marina is the nearest shelter for a visiting boat. Hazards: Swell closes the site regularly, and the ruins have sharp edges and surge in the shallows. More