Multi‐Season Geophysical Survey at the Sanctuary of Poseidon, Isthmia, Greece: GPR Investigation of Subsurface Archaeological Features 2024–2025

ABSTRACT

Ground‐penetrating radar (GPR) surveys conducted during the 2024 and 2025 field seasons at the Sanctuary of Poseidon in Isthmia, Greece, investigated unresolved questions concerning the spatial organisation, circulation routes and hydraulic and bathing infrastructure of this major Panhellenic sanctuary. Eighteen individually georeferenced survey grids were acquired across the Roman Bath complex, the East Field, the ground immediately south and east of the East Field, the interior of the Late Roman–Byzantine fortress, and several peripheral zones west of the temenos, and were integrated with archival research and targeted excavation. Rectilinear anomaly networks in the East Field indicate the continuation of buried architectural remains beyond the excavated limits, while survey within the fortress interior reveals a largely open subsurface with only sparse discrete reflectors. Most significantly, GPR anomalies associated with Roman‐period bathing infrastructure were tested through excavation in 2025, confirming a previously unknown heated bathing structure near the Northeast Gate and refining the reconstruction of an earlier bath complex north of the main Roman Bath. Taken together, the results bear on three broader questions about Isthmia: the physical correlates of the attested second‐century AD building programme, the spatial logic of a Panhellenic sanctuary as against the orthogonally planned Peloponnesian cities investigated by comparable survey, and the character of occupation within the Byzantine fortification. The study demonstrates the value of iterative, multi‐season GPR survey integrated with targeted excavation and precise Global navigation satellite system (GNSS)‐based georeferencing at complex, long‐lived sites.