Kroum Batchvarov had hardly slept in three days. It was the end of September and the University of Connecticut professor was on a research vessel exploring the Bulgarian waters of the Black Sea as a senior member of Black Sea Maritime Archaeology Project.

Batchvarov was watching images captured by a robot tethered to the ship more than a half-mile beneath the surface. It was late at night and despite being tired, Batchvarov was more than happy to stay awake. “I did not to want to go to sleep when I could be looking at shipwrecks,” he says.

Already, the expedition had been spectacularly successful, with many Ottoman Empire-era shipwrecks dating from the 16th to 18th centuries discovered in excellent condition. Now, as the robot’s camera moved over a new wreck, Batchvarov assumed it would be another ship from the same time period. But, as the images of the starboard oar of the vessel appeared on the screen before him, Batchvarov noticed something unusual. “It looked totally different,” he recalls. “Suddenly it dawned on me that this is a quarter rudder; this is a steering oar.”

A quarter rudder is an ancient type of large steering oar. To Batchvarov’s trained eye, its presence suggested the vessel beneath them was older than the other ships found to that point in the expedition — far older.

Batchvarov woke up Jon Adams, the team leader of the Black Sea project and founding director of the Centre for Maritime Archaeology at England’s University of Southampton, which partnered with the Bulgarian National Institute of Archaeology and the Bulgarian Centre for Underwater Archaeology on the project. Together, Batchvarov and Kroum watched like two kids in “a candy shop” as more images of the ancient vessel appeared.

Later research would reveal the vessel dated to the 13th or 14th century and had most likely served the Venetian Empire. For seven or eight hundred years, the transport vessel with a quarterdeck where the captain commanded a crew of 20 or so sailors had rested unseen in the darkness.

“It is a unique find because it has never been found anywhere else. We know of this type of vessel because of documentary evidence from the late Middle Ages and Renaissance period, but we had never seen one,” Batchvarov says. He adds that the ship was in remarkably good condition, joking that “the only thing missing is the skeletons of the crew.”

The find was the icing on the cake of one of the most successful archaeological surveys in history. In all, the team found more than 40 ships, including many Ottoman-era ships and several 19th-century ships. Most were remarkably well-preserved with rigging materials, ropes, tills rudders and decorative carvings still intact.

The waters of the Black Sea are mostly free of oxygen below about 500 feet, a maritime rarity that dramatically slows the rate of decay for ships lost in its cold depths. Batchvarov has long studied the region. A native of Bulgaria, he vacationed at the Black Sea as a child, learning to sail and dive at the storied body of water. Much of his professional career as a maritime archaeologist had been dedicated to unlocking its secrets — secrets that spanned the centuries. “There has been maritime traffic along the western shore of the Black Sea since the deepest antiquity,” Batchvarov says. In 2001, Batchvarov was the leader of the first team that successfully excavated a Black Sea vessel.

Evidence of trade between the civilizations of the Black Sea dates back to around 10,000 BCE. The Tartars used it to sell Christian slaves to places like Cairo, and it provided Europe with access to the Silk Road. Marco Polo would have seen ships like the medieval transport vessel discovered by the team.

Though it is the shipwreck finds that have made headlines, the primary goal of the project is to study the massive rise in water levels that occurred in the region following the last ice age, and to create a palaeoenvironmental reconstruction of Black Sea prehistory.

For the expedition, the main vessel was the Stril Explorer, a British ship with a helicopter landing pad. The vessel is equipped with some of the world’s most advanced underwater survey systems and is generally used to service the undersea pipes and other structures of the offshore oil industry.

The three-week survey in September represents the second year of the three-year project. Batchvarov will return with the team in September. He says specific goals for 2017’s survey have not been set, but will likely include return visits to some of the sites studied this year, as well as exploration of different areas. He is certain there are many shipwrecks still waiting to be found beneath the waters he played in as a child.

“We have 6,000 to 7,000 years of seafaring on this coast; we are bound to find more wrecks,” he says. In the meantime, the robust data and thousands of high-resolution images gathered in September will give researchers decades’ worth of material to study. “This is a mine of information that is going to feed generations of archaeologists,” he says.