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This is what is left of an artificial port that King Herod built at Caesarea. The site along the coast that Herod chose for the city did not have a natural harbor or port so he went to the great expense to build one. He used a new Roman invention called hydraulic concrete - that is concrete that sets under water.
The sea filled in the port with sand long ago and this is what is left of the docks which are now quite a bit inland.