Caesarea’s Mighty Harbor

Herod named the Caesarea harbor Sebastos, Greek for Augustus—an extraordinary monument to the emperor and to Herod’s patron. But the harbor also a monument to Roman engineering prowess.

The harbor was formed by two massive breakwaters or moles that extended nearly 1,600 feet into the sea from the shore and formed two basins in which ships could tie up and load or unload cargo. The longer, southern breakwater curved around to approach the shorter, northern one, which was straight (both are visible in the photo). Both breakwaters were wide enough (at 150 and 200 feet, respectively) to accommodate warehouses built on top and to give access to ships inside the harbor. The 60-foot gap between the two breakwaters served as the harbor entrance. Three large concrete islands supporting monumental sculpture—perhaps for use as navigational aids—straddled the entrance. Next to the shore of the harbor was an inner basin (at left in the photo) formed by another barrier that provided additional dock space.

Underwater excavators, such as those shown, have spent decades studying Caesarea’s harbor. Herod’s men built the breakwaters by laying a series of immense concrete blocks on the sea bottom, forming a chain of artificial islands that were then joined by more conventional masonry. This was made possible by the Roman invention of hydraulic concrete, a mixture of mortar and large particles of pozzolana (which is volcanic sand imported from Italy) pumice and lime. Being three times denser than water, the mixture could harden beneath the ocean surface.

The first step in the process of building the breakwaters was to construct 50-foot-long rectangular wooden boxes that were floated into place and then sunk, creating forms for the hydraulic concrete. Once the forms were in place on the sea bottom, concrete was poured into them through a large wooden tube with flexible leather joints. The tube was held close to the bottom of the form that the mouth of the tube would remain beneath the rising surface of the concrete, which kept the mixture from dissipating in the water before it could fill the form and harden. Several pouring tubes were probably used at once, held by people standing either on the upper level of the already-built portion of the breakwater or perhaps in barges anchored nearby (as shown in the artist’s rendition).

The concrete islands were then connected by parallel masonry walls; the open space between was left for the sea to fill with sand over the course of a few years. Once filled, the walls were topped and paved to hold the sand in position. Atop the breakwaters stood large barrel-vaulted warehouses and promenades that gave access to ships tied in the harbor.