The most impressive structure built under King Herod’s reign was the artificial harbor of Caesarea Maritima, Sebastos, in honor of the emperor Augustus (sebastos is Greek for Augustus). The structure was constructed on a shoreline devoid of any natural protective embayment, and given such unfavorable geomorphological conditions, innovative building materials and techniques were required to protect it from one of the Mediterranean’s highest-energy wave environments. As thoroughly described by the 1st Century CE Roman Jewish historian Josephus Flavius, the construction of Sebastos Harbor “…was to be brought to perfection by materials from other places, and at very great expenses” (Bergin 2018). These foreign materials were incorporated into large concrete blocks of the underwater foundations, described as “…this he effected by letting down vast stones of above fifty feet in length, not less than eighteen in breadth, and nine in depth...” (Bergin 2018), and were recently identified as pumiceous ash and tuff sourced from the Phlegraean Fields volcanic district, Italy.

Pumiceous ash and tuff are found to be the key ingredients in constructing underwater foundations in Roman masonry / construction, as they contain amorphous aluminosilicate compounds capable of reacting with lime and water to precipitate hydraulic insoluble phases with excellent mechanical properties.
This work examines Roman construction and repairs and reports their geochemical, geoarchaeological, and stratigraphic data from on-land Herod palace and underwater wave breakers at Caesarea Maritima Hydraulic mortars. The compositions of immobile trace metals Zr, Ti, and Nb show similar geochemical signals that correspond with the Vesuvian and Phlegraean Fields (Brandon et al. 2014), distinguished from proximal volcanics such as the Kerem Maharal volcano or Golan volcanism. Furthermore, its distinguished from northern Israel Herodian palace at Banias, that resemble the local volcanic sources at the Golan Heights.

Radiocarbon dating of the underwater wave breaker wooden structures show that during the 2nd century CE, Roman concrete was placed, presumably outside of the Herodian layout of the original wave breaker, to facilitate later repairs. The repairs include mortars with evidence for hydraulic reactions based on high-resolution chemical and structural data. In addition, on-land mortars from Herod's Promontory Palace show the presence of pozzolana from the Vesuvian and Phlegraean Fields, suggesting the volcanic materials were used for functions beyond the underwater construction. These observations provide the first archaeological evidence for continuity in importing volcanic ash, i.e., pozzolana, from the area of Naples in Italy, during and after the Herodian construction.
Scholars rely on the writings of Josephus Flavius as the starting point for information about Herod the Great. Recent archaeological excavations in Caesarea Maritima allowed material scientists, archaeologists, and geochemists to find the links between historical texts and Roman concrete in the Sebastos Harbor.

References
Bergin, B., 2018. The innovative genius of Herod at Caesarea Maritima. Cultural and religious studies, 6(7), pp.377-390.
Brandon, C.J., Oleson, J.P., Jackson, M.D. and Hohlfelder, R.L., 2014. Building for eternity: the history and technology of Roman concrete engineering in the sea.