Archaeologists were digging in the old city of Ur, where Abram came from (later known as Abraham), when they found many tablets which the Babylonians had written on. Some of these tablets dated to the time of Nabonidus, and they described the fact that Nabonidus, king of Babylon, had left the country for several years, leaving the city in the care of his son.
His son's name was Belshazzar, the same man who was killed by the Medes and Persians on the night the writing appeared on the wall.