Snorkelers off the Greek island of Zakynthos have for years brought up stone disks, tubes, and ring-shaped objects from shallow water, convinced they had found the remnants of an ancient city lost to the sea. The formations sit at a depth of just 2 to 5 meters in Alikanas Bay, close enough to shore that recreational divers stumble on them regularly. But peer-reviewed geochemical analysis has identified every one of those objects as a natural product of ancient methane seeps, not human craftsmanship, raising a pointed question: how many other supposed underwater ruins around the world are the same kind of geological false positive?
Why methane-seep carbonates keep fooling divers
The confusion at Zakynthos is not random. The formations display pipe-like, disk, and doughnut morphologies that closely resemble column bases, paving stones, and architectural fragments. Divers who encounter them in murky, shallow water have no easy way to distinguish microbially produced rock from hand-carved stone. Greece’s Ephorate of Underwater Antiquities examined the site in person and initially treated the discovery as a potential archaeological find before the geological explanation emerged.
That explanation centers on a well-documented chemical process. When methane gas migrates upward through sediment and meets sulfate-rich seawater, microbial communities oxidize the methane and trigger the precipitation of calcium carbonate. The result is hard, sometimes elaborately shaped rock that forms around gas conduits and along fracture planes. Research on methane seep communities has established that carbonate-hosted methanotrophy occurs at deep-sea sites worldwide, confirming that the underlying chemistry is not limited to any single location or depth range.
The pattern creates a predictable class of false positives. Shallow coastal zones with hydrocarbon reservoirs beneath the seabed are especially prone to producing these formations in waters where divers are most active. Because the shapes mimic architectural elements, the misidentification cycle repeats: discovery, excitement, media coverage, and then, eventually, laboratory testing that rules out human origin. The Zakynthos case is the best-documented example, but the same morphological traits appear at methane-seep sites across the Mediterranean and beyond.
Geochemical proof from Alikanas Bay
The definitive study on the Zakynthos formations was published in Marine and Petroleum Geology. Researchers conducted field surveys and collected samples from the 2 to 5 meter depth site in Alikanas Bay, then subjected them to isotopic analysis. The results showed that the objects are exhumed hydrocarbon-seep carbonates, meaning they formed underground through methane-driven mineralization and were later exposed on the seafloor by erosion. No tool marks, no cultural artifacts, and no construction materials appeared anywhere in the samples.
The isotopic signatures were the deciding factor. Carbon isotope ratios in the concretions matched values typical of methane-derived carbonates, not the ratios found in limestone quarried or shaped by ancient builders. The morphologies that looked so convincing underwater turned out to reflect the geometry of gas migration pathways: cylindrical pipes where methane rose through narrow channels, flat disks where it spread laterally along bedding planes, and ring shapes where it vented around a central conduit.
Greece’s Ephorate of Underwater Antiquities accepted the geological assessment after its own in-situ examination of the site. That institutional sign-off effectively closed the archaeological question for Zakynthos, but it also highlighted how long the misidentification persisted before laboratory work settled the matter. For several years, local tour operators and enthusiasts promoted the formations as ruins, illustrating how quickly a narrative of “lost city” can take hold when visual impressions go untested.
How verified underwater ruins actually get confirmed
The Zakynthos case gains sharper focus when set against a site where submerged ruins are real. At the Baia Archaeological Park off the coast of Italy, Roman-era structures sit on the seafloor because volcanic activity caused the land to sink. Italian authorities have systematically mapped Baia’s seafloor using acoustic and photographic surveys, producing detailed inventories of walls, mosaics, and statuary that are unmistakably human-made.
The difference in verification protocols is stark. At Baiae, state agencies assigned protected status, established monitoring programs, and integrated conservation planning into coastal management. Archaeologists work alongside marine geologists and conservation divers, and the resulting maps show coherent street plans, building outlines, and decorative elements that fit securely into known Roman architectural styles.
At Zakynthos, by contrast, the seafloor objects are scattered, lack any coherent urban layout, and show no associated cultural material such as pottery, metal fittings, or worked stone blocks. Once geochemical tests demonstrated a methane-seep origin, there was no basis for assigning archaeological protection. The gap between the two sites illustrates a practical problem: without standardized screening for underwater discoveries, each new find of unusual stone shapes on the seafloor can trigger the same cycle of premature claims and delayed correction.
Open questions about seafloor misidentification
Several gaps in the evidence remain. No primary field logs or diver-submitted coordinates from the earliest discovery claims at Alikanas Bay have surfaced in public records, making it difficult to reconstruct how interpretations evolved dive by dive. Informal photographs circulated online long before scientists sampled the site, but those images rarely included scale bars, depth readings, or precise locations. That lack of basic contextual data almost certainly prolonged the confusion.
The Ephorate’s confirmation of its on-site visit appears mainly in secondary reporting, with limited detail about the criteria officials used in the field. It is unclear, for example, whether they initially relied on visual inspection alone or consulted geologists before announcing their preliminary views. The subsequent Marine and Petroleum Geology study filled in the geochemical picture, but the administrative decision-making process that bridged local rumor and scientific publication remains largely undocumented.
Those uncertainties matter beyond Zakynthos. Around the world, coastal communities and dive operators have strong incentives to frame unusual seafloor features as heritage attractions. In the absence of clear protocols, visually striking but natural formations can be branded as “sunken temples” or “ancient harbors,” drawing visitors and media coverage long before specialists weigh in. When later analyses overturn those claims, public trust in both archaeology and geology can suffer.
Researchers who study methane seeps argue that better communication could reduce these misfires. Because seep-related carbonates tend to form in recognizable morphologies-tubes, domes, pavements, and ring structures-training materials for dive guides and local heritage officials could flag those shapes as candidates for geological explanation. Simple checklists could encourage early documentation of coordinates, depths, and surrounding sediments, creating a baseline record even before professional teams arrive.
The Zakynthos case also underscores the value of integrating geochemical screening into underwater cultural heritage assessments. Carbon and oxygen isotope analyses are relatively routine in marine geology labs and can quickly distinguish between methane-derived carbonates and typical building stones. Making such tests a standard step for ambiguous finds would help filter out geological lookalikes before they are promoted as lost cities.
At the same time, scientists caution against swinging too far in the opposite direction. Genuine submerged sites like Baiae demonstrate that real underwater ruins do exist and can be extensive, complex, and historically important. Coastal subsidence, sea-level rise, and tectonic activity have all drowned once-inhabited shorelines. The challenge is not to dismiss every odd rock as natural, but to apply consistent, multidisciplinary scrutiny-combining morphology, context, artifact associations, and geochemistry-before assigning cultural significance.
In that sense, the “ghost city” of Zakynthos may yet prove useful. As a well-documented example of how methane-seep carbonates can masquerade as architecture, it offers a template for refining field practices and public messaging. Future divers who surface with tales of columns and pavements in shallow bays may still be right-but thanks to the lessons from Alikanas Bay, they will have a clearer path to finding out.
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*This article was researched with the help of AI, with human editors creating the final content.
