Dundee City Council staff discovered the massive "castle rock" built into the basement of the former Kennedy's Steak House building.
The huge rock formation stretched back up the hill towards High Street and was the base for the city's first castle that lends the street its name.
Lynne Short convener of Dundee City Council's city development committee said: "In light of the interest in the city's underground with the recent opening up of the Vaults and our wider heritage in general, it's amazing to find out that the very rocks the city of Dundee was first built on are still there!
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"The building created around it has an interesting enough history in itself as it was the only one left standing in the area during the early 20th century when there was a massive clearance to make way for the Caird Hall complex.
"Castle Street was cut through the vast formation on which the ancient Castle of Dundee stood but clearly large chunks of the rock remained under this building when it was put up."
Dundee Town Council took ownership of the site in 1914 as part of the Central Improvement Scheme when many buildings were earmarked for demolition to make way for the Caird Hall.
Contemporary photographs held in the City's Archives show that 31-33 Castle Street was not demolished. The sandstone building was annexed and included in the design and construction of the new Caird Hall complex and received a fresh, modern appearance. In 1994 it was given a Category B listing.
Renovation has been completed on the retail unit and it will be available for let in a few weeks.
(CM/CD)
