Inspired by local rumours, an archaeologist from Van Yüzüncü Yil University on the shores of the lake teamed up with three independent divers to make the find.
They believe that the remains date back to the ancient nation of Urartu, which encompassed what is today Turkey, nearby Iran and Armenia.
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The divers have spent some 10 years exploring the lake, known by visitors for its striking turquoise waters and a reputed monster.
In that time they had come across a Russian shipwreck and large microbialites (natural accreted structures) but archaeological remains are a first.
Lead diver and photographer Tahsin Ceylan told Turkish press that the fortress covered an area of 100 hectares, with neatly cut stone walls reaching 3-4m high, though the team didn’t know how much more of the structure might be buried.
The alkaline waters of the lake had served to preserved the exposed sections.
The team now plans to continue surveying the ancient underwater remains with a view to excavation.