The Assyrian Empire lasted for about 600 years. Around 600 BC, before the people of ancient Mesopotamia were absorbed into the great Persian Empire, the last Assyrian king started a project. He began collecting a library of clay tablets of all the literature of ancient Sumer, Babylonia, and Assyria.
When archaeologists discovered the library at Nineveh in the 1850s, they found over 30,000 clay tablets written in cuneiform with different stories, histories, magical texts, letters, medical texts, government documents and fragments of documents. These tablets are our single most important source of knowledge about ancient Mesopotamia. They are still being researched today. What a find!