Observatory Exterior

Confident estimate: this is an astronomical observatory. It is hollow and must have an entrance.

Structure age: twelve hundred years (inferred from chemical weathering of granite).

Composition: cut granite blocks and concrete. Banding on the granite suggests it was hauled into place by a crane or lift bags.

Structure surface coating: silicon bronze. The coating was cold-applied by wrapping the structure in a net of wires, then electrifying them and introducing silicon and copper feedstock. The coating electroformed over the stone.

Induction: the builders had access to electrical power, but not to the hot forging techniques used to smelt large bronze plates. This could be the result of technological development underwater.

Signs of activity: a gold coating over the bronze has been chipped away over the past several hundred years. Tool marks suggest scavenging or salvage.

Observation: if this is an observatory, the top of the structure should break the surface to allow undistorted imaging of the sky. It does not.

Conclusion: the planetary sea level rose rapidly during the past 1200 years.