UNDERWATER ARCHAEOLOGICAL EXPLORATIONS IN THE AQUATORY OF MESAMBRIA (Nayden Prahov – naydenprahov@yahoo.com, Kalin Dimitrov) Fifteen underwater archaeological expeditions were carried out between 1960 and 1984. Fortification structures were discovered to the north and northeast of the peninsula. In 2017, explorations were carried out from the northeastern side of the peninsula. Structures that were known from the previous explorations were rediscovered: a section of Late Antique fortification wall built of stones bonded with mortar, over 2.20 m wide, c. 15 m long; a rectangular tower, 6 m by 8 m in size, built of ashlars, some of them over 1 m long; a section of fortification wall built of ashlars, c. 2 m wide, c. 50 m long, situated to the east of the tower; a fortification wall built of ashlars, situated at c. 20 m to the east of the previous one; a section of fortification wall built of stone slabs, c. 2.80 m wide, c. 40 m long, with a rectangular tower, 5.50 m by 6 m in size. Sections of two new fortification walls were documented, the first one built of stone slabs, c. 1.80 m wide, and the second one in ¬_opus mixtum_. Underwater archaeological surveys were carried out to the northwest of the peninsula in the area where structures of the Late Classical and Early Byzantine periods were known. Sections of a Late Antique fortification wall built in ¬_opus mixtum_, c. 3 m wide, were documented.