A group of maritime archeologists studying sea levels in the Black Sea have uncovered 41 shipwrecks this year as a “complete bonus.”

The Black Sea Maritime Archeology Project has been trawling the seabed to understand how quickly the water level rose after the last Ice Age, 20,000 years ago. But their surveys ended up uncovering dozens of previously unknown wrecks. Many of the discoveries are in excellent condition, thanks to low oxygen levels below 150 meters, which slows decay.

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“The wrecks are a complete bonus, but a fascinating discovery, found during the course of our extensive geophysical surveys,” said Jon Adams, a University of Southampton maritime archaeologist and principal investigator of the project, in a statement.

The team took thousands of still photographs of the shipwrecks, and then used photogrammetry, a technique that uses software to calculate the positions of millions of points in space, to build 3D models of the discoveries.

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Some of the ships discovered are thousands of years old, dating to the Byzantine empire, while others are from the Ottoman period.

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Hulls, masts, tillers, and rudders are all clearly discernible. “Certainly no one has achieved models of this completeness on shipwrecks at these depths,” said Adams.

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This article originally appeared on Quartz.