Sunken Fortress from Ancient Thrace Discovered at Bulgaria’s St. Thomas Island Which Used to Be Black Sea Peninsula

Sunken Fortress from Ancient Thrace Discovered at Bulgaria’s St. Thomas Island Which Used to Be Black Sea Peninsula

A diver from the underwater archaeology expedition shows one of the stone blocks that made up the outer wall of the sunken fortress on what is today St. Thomas Island off the coast of Bulgaria’s Primorsko. Photo: National Museum of History

A now sunken fortress from Ancient Thrace has been discovered at the tiny St. Thomas Island (also known as Snake Island) off Bulgaria’s Black Sea coast during an underwater archaeology expedition, which has also found that the island used to be a peninsula before the Middle Ages.

Before the discovery of the sunken fortress, on the island itself, the archaeologists found the ruins of an Ancient Thracian settlement with ritual pits, an Early Byzantine settlement, and a monastery from the time of the Second Bulgarian Empire (12th – 14th century).

The St. Thomas Island is located off the coast of the Bulgarian Black Sea resort of Primorsko (which more recently made headlines with the discovery of the Ancient Thracian Primorsko Gold Treasure).

The St. Thomas Island in the Black Sea has a total territory of 0.012 square kilometers (12 decares, or 3 acres).

It lies only 0.2 nautical miles (appr. 370 meters) from the mainland, and is located north of the mouth of the Ropotamo River, in the Bay of Arkutino, and is actually part of the Ropotamo Natural Preserve.

It also known as the Snake Island (not to be confused with the Ukrainian Snake Island (or Serpent Island) in the Northern Black Sea) because of the large number of grey water snakes that inhabit it.

Bulgaria has only several tiny islands off its coast in the Black Sea (six, seven, or eight depending on how the definition of an island, and whether islands now artificially connected to the mainland are counted.

Many of those were seemingly former peninsulas, and pretty much all of Bulgaria’s Black Sea coast boasts stunning underwater archaeology remains precisely thanks to the partial or full sinking of the coast in the Prehistory, Antiquity, and Middle Ages.

St. Thomas Island (Snake Island) lies only about 400 meters off Bulgaria’s Blacks Sea coast. Photos: Sozopol Municipality, Wikipedia

Otherwise, the most notable of Bulgaria’s Black Sea islands in recent years have been the St. Ivan Island off the coast of Sozopol, which is known for its Early Christian / Early Byzantine monastery where in 2010 relics of St. John the Baptists were discovered, and the St. Anastasia Island near Burgas with its late medieval monastery.

Another intriguing underwater archaeology story recently connected with the Black Sea has been the hypothesis that a large sunken island existed in its southwestern part, near today’s coast of Bulgaria and Turkey.

A 2,000-year-old sunken Ancient Roman ship was recently discovered on the bottom of the Black Sea in Bulgaria’s territorial waters by the international Black Sea M.A.P. expedition, alongside several dozen other ancient, medieval, and Modern Era shipwrecks.

The newly discovered Ancient Thracian sunken fortress at the St. Thomas Island adds another great underwater archaeology landmark to the Bulgarian Black Sea coast.

The sunken fortress has been discovered during an underwater archaeology expedition in the waters around the small island led by Prof. Ivan Hristov from Bulgaria’s National Museum of History in Sofia.

Hristov is a long-time researcher of the archaeological sites along Bulgaria’s Black Sea coast, including a number of fully or partly sunken settlements and fortresses.

He recently published a book based on 8 years of field research of fortresses and settlements on the southern Black Sea coast of today’s Bulgaria.

The book is entitled “Mare Ponticum. Coastal Fortresses and Harbor Zones in the Province of Haemimontus, 5th – 7th Century AD”, and looks at the Haemimontus province of the Early Byzantine Empire in the Late Antiquity and early Middle Ages.

An underwater archaeology expedition led by Hristov around the St. Thomas Island earlier in August 2018 has discovered two zones around the island with archaeological remains, the National Museum of History in Sofia has announced.

The first zone is located right southwest of the island, between the island and the mainland, at a depth of only 1 meter (appr. 3 feet).

In addition to the pottery fragments scattered there, Hristov and his team of divers has found there a large number of stone blocks seemingly more or less shapes as cuboids, and showing clear traces of having been processed by humans.

The St. Thomas Island is connected to the Black Sea coast of the Bulgarian mainland with a shallow underwater reef at a depth of only 2 meters (appr. 6 feet).

The special boat provided for the underwater archaeology expedition at the St. Thomas Island by Bulgaria’s Center for Underwater Archaeology in Sozopol is packed with scanning equipment. Photo: National Museum of History

The stone blocks from the sunken fortress form a line between the island and the mainland that is perpendicular to the reef.

That appears to have been part of the sunken fortress’s outer wall fencing off a former Black Sea peninsula, now the St. Thomas Island, from the mainland.

A number of Bulgaria’s other surviving small Black Sea peninsulas and capes were transformed into fortresses that way as early as the early Antiquity period: all the way from Cape Kaliakra with its Kaliakra Fortress in the north to the Talaskara Fortress on Cape Chervenka, also known as the Chrisosotira (“Golden Savior, Golden Christ”) Peninsula, and the Urdoviza (Urdovisa) Fortress near today’s town of Kiten, down to the border with Turkey.

The discovery of the submerged fortress wall, presumed to be of Ancient Thracian origin, means that the St. Thomas Island was once both a peninsula, and a now sunken fortress.

“The [stone] blocks are found perpendicular to the underwater reef, and are part of a structure that is severely demolished by the sea storms which [used to] fence off today’s island from its western side which used to be the side from which the former peninsula was accessible from the mainland,” Bulgaria’s National Museum of History explains.

“Apparently, this was a previously unknown sunken fortress wall [that was] the work of the Ancient Thracians,” the Museum points out.

The zone around the St. Thomas Island where the underwater archaeology expedition has found archaeological remains is located south of it.

There at a depth of 9 meters (appr. 30 feet), the divers have found part of a Late Antiquity amphora, and limestone blocks, or bricks, which seem to have been intended for an ancient building on the island, former peninsula.

“The archaeologists are positive that the island used to be a peninsula in the past,” the Museum states.

“Evidence of that are the measurements of the clearly visible wide reef connecting today’s small Black Sea island to the mainland,” it adds.

“As was mentioned, the deepest section, which is found in the middle, between the island and the mainland, is only 2 meters (6 feet) deep. Given the general rise of the Black Sea waters level between the 7th century AD and the 12th century AD, the large isthmus connecting the St. Thomas Island with the mainland was submerged,” the Museum elaborates.

It stresses the fact that all now submerged archaeological structures in what was the coastal of early Byzantium’s Haemimontus province in the 5th – 7th century AD have been found to lie at a depth of between 0.5 and 1.4 meters (2 – 5 feet).

Submerged settlements and fortresses from the Antiquity, and more specifically, the Hellenistic and Classical Age, however, can be found along Bulgaria’s Black Sea coast at depth of 6 to 8 meters (20 – 25 feet)

“These estimates are indicative of the entirely different coast line [in Bulgaria’s Black Sea section] in ancient times compared with today’s coast line, and the comparatively larger land territory of the archaeological sites from the Late Antiquity, including with respect to the newly explored island [of St. Thomas],” Bulgaria’s National Museum of History stresses.

The underwater archaeology expedition that has found the sunken fortress at the St. Thomas Island led by archaeologist Ivan Hristov is funded by Bulgaria’s Ministry of Culture.

The expedition included Kiril Velkovski, a geophysicist from the Center for Underwater Archaeology in Sozopol (part of the National Institute and Museum of Archaeology in Sofia), Totyu Angelov, a geodesist from the Kartproekt firm, and a team of divers from the Municipal Diving Center in Sozopol led by Tencho Tenev.

Lead archaeologist Ivan Hristov poses in diving gear during the underwater archaeology expedition around the St. Thomas Island off Bulgaria’s Black Sea coast. Photo: National Museum of History

Diver Tencho Tenev poses with the St. Thomas Island visible in the background on the right, and the Bulgarian mainland on the left. Photo: National Museum of History

The underwater research around the St. Thomas Island has been performed in continuation of archaeological digs carried out by Hristov’s team on the island itself back in June 2018, during which the archaeologists found an Ancient Thracian pit shrine, Late Antiquity structures, and a medieval monastery.

The methods used for the archaeological research on and around the St. Thomas Island in 2018 follows the model for maritime archaeology sites used by the National Museum of History in Sofia and the Center for Underwater Archaeology in Sozopol, in which the archaeologists start on land, and they continue their exploration in the nearby waters.

For its underwater exploration around Bulgaria’s St. Thomas Island, the team has been supplied by Kalin Dimitrov, Director of the Center for Underwater Archaeology, with a special boat equipped with the latest scanning technology. The boat has been operated by sea captain and diver Radostin Penev, a long-term diver in Scandinavia presently employed by the Sozopol Center.

The underwater archaeological exploration around the St. Thomas Island, which led to the discovery of the sunken fortress from Ancient Thrace, encompassed all sides of the island, down to a depth of 15 meters (appr. 50 feet).

Also check out these stories about sunken or submerged cities along Bulgaria’s Black Sea coast and beyond:

Archaeologists to Resume Excavations of Half-Sunken Ancient Black Sea City Byzone near Bulgaria’s Kaliakra Cape after 10-Year Break

Maritime Archaeologists Find Bronze Age Settlement under Black Sea’s Seabed off Bulgaria’s Coast

Early Byzantium’s Haemimontus Province on Bulgaria’s Southern Black Sea Coast Presented in New Book Based on 8 Years of Excavations

Submerged Ancient Thracian Capital Seuthopolis in Bulgaria’s Koprinka Water Reservoir Could Be ‘Resurfaced’ with US Government Money

‘Resurfacing’ of Submerged Ancient Thracian Odrysian Capital Seuthopolis Could Make It Global Tourist Attraction, Archaeologist Says

Also check out these stories about the findings of the Black Sea M.A.P. underwater archaeology expedition:

Archaeologists Discover Perfectly Preserved 2000-Year-Old Roman Ship, 20 Other Shipwrecks in Black Sea Off Bulgaria’s Coast

Pre-Columbian Mediterranean ‘Round’ Ship Discovered for the First Time by Underwater Archaeology Expedition in Bulgaria’s Black Sea Zone

No ‘Biblical Deluge’ but Gradual Ice Age Melting Made Black Sea ‘a Sea’, Archaeologists Find after Underwater Expedition in Bulgaria’s Waters

And these stories about sunken ancient ships elsewhere in the Black Sea:

Sunken Roman Ship from 2nd-3rd Century AD Discovered in Black Sea off Crimea’s Coast

2,500-Year-Old Sunken Ancient Greek Ship Used to Sail to Crimea Discovered in Ukraine’s Black Sea Waters

Large Sunken Byzantine Ship Discovered in Black Sea off the Coast of Sevastopol on Crimean Peninsula


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