UNDERWATER ARCHAEOLOGICAL EXPLORATIONS IN THE AQUATORY OF MESAMBRIA (Nayden Prahov – naydenprahov@yahoo.com, Kalin Dimitrov) During underwater diving surveys in the southern gulf of the peninsula, sectors of the early Byzantine fortification wall were discovered, built of stones bonded with mortar. A stone cylindrical segment of a column was discovered close to the coast. The rectangular fortification tower in the eastern gulf was cleaned. It was built of ashlars and measured 8 m by 8 m. The fortification wall to the east of the tower was also cleaned. It was situated at 2 – 4.50 m in depth and its inner and outer faces were built of ashlars. The wall was clearly visible at 35 m in length. Two monumental structures of piled stones were documented. The first one had a L-like layout and was c. 19 m wide, c, 3.50 m high and c. 35 m long. The second structure was c. 75 m long, up to 7 m wide and c. 1 m high. Probably, the structures were ancient breakwaters. Underwater diving surveys and excavations were carried out to the west of the Argirova Breakwater. Sectors of the Early Byzantine fortification wall, built in opus mixtum and c. 3 m wide, were discovered parallel to the west of the structure of piled stones that was c. 60 m long. The geophysical explorations in the water area to the south of the peninsula continued; multi-beam echo-sounder and lateral scanning sonar were applied.