Nineveh was the capital of the empire of Assyria. It now lay in ruins on the banks of the Tigris river in what is now modern day Mosul, Iraq.
The Assyrian leader Sennacherib made Nineveh a great city. Since the city was located at the junction of the Tigris and Khosr Rivers, it was located on an ideal highway between the Indian Ocean, and the Mediterranean Sea. This meant that wealth flowed from many sources, making it a great city in the days of the prophet Jonah.
Nineveh fell to the Babylonians and Susianians in 612 BC, and was pretty much abandoned in the year 400 BC as mentioned by the greek historian Herodotus. The city of Mosul is located on the opposite side of the river, and has been much more important ever since.