The appearance of the ruins on the banks of the Tigris River triggered an archaeological dig, with a joint Kurdish and German team keen on surveying the treasures of the complex, known as Kemune.
The site of Kemune can be dated to the time of the Mittani Empire, and is estimated to be nearly 3,500 years old.
The Mittani Empire is one of the least-researched empires of the Ancient Near East.
It spanned from the eastern Mediterranean coast to the east of present-day northern Iraq.
Archaeologists first discovered Kemune in 2010, when water levels in the reservoir were low, but its recent emergence marked the first time excavation at the site became possible.
During their survey, the Kurdish-German team found an array of items, including large fired bricks which were used as floor slabs and remains of wall paintings in bright shades of red and blue.
Now, it is submerged again, with little clarity over when the site may reappear. For the moment, at least, the past has receded from view as the water level in the Mosul Dam rises again.