The Underground City of Byzantius is located in Turkey.
The Scooby-Doo Show
The gang heads to Turkey to visit their friend, Dr. Brixton, who has discovered a buried city that has remained untouched for thousands of years. Dr. Brixton informs the gang that his workers were all frightened off by a tar monster, meaning the digging had be postponed. The underground city was said to be guarded by the monster, who protected the ancient treasure. (The Tar Monster)