It seems commonly accepted that Xenophon saw the ruins of Nineveh during the Ten Thousand Retreat.
In the days of the Greek historians Ctesias and Herodotus, 400 BC, Nineveh had become a thing of the past; and when Xenophon the historian passed the place in the Retreat of the Ten Thousand the very memory of its name had been lost. It was buried out of sight (Wiki)
But why is it assumed that what he saw was Nineveh specifically? Is it really that unambiguous from Xenophon's text?
From this place they marched one stage, six parasangs, to a great stronghold, deserted and lying in ruins. The name of this city was Mespila and it was once inhabited by the Medes. The foundation of its wall was made of polished stone full of shells, and was fifty feet in breadth and fifty in height. Upon this foundation was built a wall of brick, fifty feet in breadth and a hundred in height; and the circuit of the wall was six parasangs. Here, as the story goes, Medea, the king's wife, took refuge at the time when the Medes were deprived of their empire by the Persians. To this city also the king of the Persians laid siege, but he was unable to capture it either by length of siege or by storm; Zeus, however, terrified the inhabitants with thunder, and thus the city was taken.