Heritage developer Lawrie Forbes said crockery, glass and ceramics had been found under the building's floor on land that was the site of a fire that burned down several houses back in 1868.
Mr Forbes had archaeologist Phoebe Scrivener on site as worker Alan Bryan lifted a section of the asphalt floor of a building Mr Forbes plans to turn into an arts hub.
The ground underneath was black and had a burned smell.
Mr Forbes said the 1868 fire had not killed anyone, but had cleared land for the Athenaeum, which opened in 1870, to be built.
Mr Forbes had, however, got to the bottom of why apart of the building with no loading on it had subsided.
The ground underneath was ``grey puggy clay''.
There were ``fingers'' of such earth, along with more solid ground, under much of the central city.
``A lot of Dunedin is built on it.''
Engineers would now come up with a solution for the issue of the clay.
Much like the warehouse precinct, Mr Forbes said, ``until you dig down and find out what's below'' it remained an unknown.
