Caesarea Underwater Harbor (Sebastos)

Demonstrates Herod the Great's extraordinary engineering ambitions and provides the earliest known large-scale use of hydraulic concrete in an open-sea setting, predating similar Roman projects in Italy.

The underwater harbor of Caesarea Maritima, known in antiquity as Sebastos (the Greek equivalent of Augustus, in honor of Caesar Augustus), represents one of the most remarkable engineering achievements of the ancient world. Constructed by Herod the Great between approximately 22 and 10 BCE along an otherwise harborless stretch of the Mediterranean coast, the artificial harbor transformed a minor anchorage called Strato's Tower into the principal port of the eastern Mediterranean.

The primary ancient source for the harbor's construction is Josephus, who describes it in detail in both Jewish Antiquities (15.331-341) and Jewish War (1.408-414). Josephus marvels at the harbor's size, comparing it favorably to the Piraeus at Athens. He describes massive breakwaters built in water up to 20 fathoms (about 36 meters) deep, towers flanking the harbor entrance, warehouses, and a temple of Augustus visible from the sea.

Modern investigation of the submerged harbor began in 1960 when underwater pioneer Edwin Link conducted the first survey using sonar and diving. He identified the general outline of the harbor beneath the waves and confirmed that substantial structures survived on the seabed. The systematic study of the harbor began in the 1980s with the Combined Caesarea Expeditions and the Caesarea Ancient Harbour Excavation Project (CAHEP), involving international teams under Robert Hohlfelder, John Peter Oleson, Avner Raban, and others. These projects combined underwater archaeology, marine geophysics, and materials science to reconstruct the harbor's construction, use, and eventual failure.

The most revolutionary discovery concerned the construction technology. Analysis of cores taken from the submerged breakwaters revealed that Herod's engineers used hydraulic concrete, specifically Roman pozzolana imported from the Bay of Naples area. Pozzolana is volcanic ash that, when mixed with lime and seawater, produces a concrete that sets and hardens underwater. This was the first identified large-scale use of hydraulic concrete in an open-sea setting. The breakwater blocks were enormous: wooden formwork was floated into position, sunk, and then filled with the pozzolana mixture, which hardened on the seabed. Some individual poured blocks measured over 11 by 15 meters. The logistics of importing the volcanic ash from Italy, transporting it to the Levant, and deploying it on this scale indicate an engineering project rivaling any in the Roman world.

The harbor had two main breakwaters. The southern breakwater extended approximately 600 meters from shore in a curving arc. The northern breakwater was shorter, about 275 meters, creating a harbor entrance roughly 20 meters wide facing northwest. The total enclosed area was approximately 3.5 hectares (about 8.5 acres). Inner harbor basins, quays, and warehouse foundations have been mapped on the seabed.

A lighthouse, modeled on the Pharos of Alexandria, stood at the end of the southern breakwater. Josephus mentions three colossal statues at the harbor entrance. Flushing channels were built into the northern breakwater to allow wave action to push seawater through the harbor, preventing the accumulation of silt and maintaining depth for shipping. This was a sophisticated solution to one of the main challenges of ancient artificial harbors.

Despite this engineering brilliance, the harbor ultimately failed. Geological and archaeological evidence shows that within a few decades of construction, the breakwaters began to subside and break apart. The cause was geological: the harbor was built on an active fault zone, and tectonic activity caused differential settlement of the massive concrete structures. Cores from the breakwaters show cracking and displacement that began relatively soon after construction. By the late 1st century CE, portions of the harbor were already problematic, and by the Byzantine period, the original Herodian harbor had largely sunk below functional levels. Later occupants built smaller harbor facilities inshore of the original.

The harbor was the port from which Paul sailed and at which he arrived on multiple occasions described in Acts. Paul was taken from Jerusalem to Caesarea under military escort after the conspiracy against his life (Acts 23:23), was imprisoned there for two years under Felix and Festus (Acts 24-26), and finally sailed from Caesarea to Rome for his trial before Caesar (Acts 27:1). The harbor Paul used would have been the Herodian facility, though by his time (c. 57-59 CE) it may already have shown signs of deterioration.

Current research includes ongoing mapping of the submerged remains using photogrammetry, side-scan sonar, and sub-bottom profiling. The materials science of the Roman concrete continues to attract interest from modern engineers studying durable marine construction materials. In 2017, researchers published studies showing that the Roman concrete actually grew stronger over time as seawater percolated through it, creating mineral crystals that reinforced the matrix.

The underwater site is part of the Caesarea National Park and is accessible to scuba divers. An underwater archaeological park allows visitors to follow a marked trail among the submerged ruins. Surface finds and construction materials are on display at the Caesarea visitor center and the Hecht Museum at the University of Haifa.

• Earliest known large-scale use of Roman hydraulic concrete (pozzolana) in an open-sea construction, with volcanic ash imported from Italy
• Massive breakwaters with individual poured concrete blocks measuring over 11 by 15 meters
• Innovative flushing channels to prevent silting, showing sophisticated understanding of coastal hydrology
• Evidence of geological failure due to tectonic subsidence beginning within decades of construction
• Harbor entrance approximately 20 meters wide with lighthouse modeled on the Pharos of Alexandria
• Total enclosed harbor area of approximately 3.5 hectares, confirming Josephus's description of its enormous scale

Caesarea Maritima and its harbor feature extensively in the Book of Acts as the primary port city of Roman Judea. The centurion Cornelius, the first Gentile convert, was stationed at Caesarea (Acts 10:1). Peter had his vision of the sheet with unclean animals at Joppa and then traveled to Cornelius at Caesarea, where the Holy Spirit fell on Gentile believers for the first time (Acts 10:24-48). Paul passed through the harbor of Caesarea multiple times. After his conversion, he was sent from Caesarea to Tarsus for safety (Acts 9:30). He landed at the harbor returning from missionary journeys (Acts 18:22). He stayed with Philip the evangelist at Caesarea (Acts 21:8). Most significantly, Paul was imprisoned at Caesarea for two years and made his defense before Felix, Festus, and King Agrippa there (Acts 23-26) before sailing from this harbor to Rome. The harbor Herod built was thus the physical setting for pivotal moments in early Christian history, from the first Gentile conversions to the beginning of Paul's journey to Rome.