Bulgarian archeologists study the remains of a mediaeval customs where the Byzantine Empire has paid the taxes it owed to the Bulgarian tsars for more than 200 years. The site is in the vicinity of tiny town Debelt, 14km into the mainland from Bulgaria's major southern coastal city Burgas.

According to a recent hypothesis, this is the place where Bulgaria's first Christian ruler -- Prince Boris I -- adopted the new faith. "There are written mediaeval recors to confirm that," Dr. Peter Balabanov who leads the expedition said.

He revealed that within the boundaries of the customs, his team discovered a Christian church, which dates back to the first half of the ninth century A.D. The dating coincides with the time of the Christian conversion of the Bulgarians.

"We believe that this church and the seals of Prince Boris which were discovered there indirectly confirm our hypothesis," Dr. Balabanov explained.

In middle age, Debelt was a port as several rivers connected the nearby Mandrensko lake with the Black Sea. Later the sea level went 6 to 8 meters up and the flooded area around Debelt turned into a huge swamp which was drained in the 20s of last century.