Pattern is only the beginning
Sonar, photogrammetry, lidar, magnetometry and diver survey can reveal anomalies or geometric patterns. Those methods are excellent for finding targets, but an image that looks architectural is a hypothesis generator rather than proof of construction.
Context turns a shape into archaeology
Researchers look for worked material, artefact scatters, repeated structural relationships, occupation surfaces, tool marks, human remains, datable deposits or construction sequences that are hard to reproduce through natural processes.
Geology must be tested, not ignored
Bedrock jointing, erosion, collapse and sedimentary bedding can imitate walls, steps and paving. Serious investigations map the local geology and ask whether the proposed structure fits natural fracture orientations or requires human modification.
Documentation matters because excavation destroys context
UNESCO's professional rules require project design, qualified archaeologists, conservation planning and thorough documentation. Underwater sites are fragile, and removing objects without recording their spatial and sedimentary relationships can destroy the strongest evidence.
Research record
UNESCO — Convention on the Protection of the Underwater Cultural HeritageInternational standard · open sourceUNESCO — Annex to the 2001 ConventionProfessional rules · open sourceUNESCO — Emergency preparedness and underwater archaeologyProfessional guidance · open sourceHow Do Archaeologists Know a Submerged Site Is Human-Made?
Direct answer: Archaeologists identify a submerged site as human-made when multiple independent lines of evidence converge. Geometry alone is weak; artefacts, context, modification, stratigraphy, dating and geological exclusion make the case.
The evidential threshold underwater should be at least as high as on land. 'It looks artificial' is an observation, not an archaeological conclusion.