Structures > Etemenanki
By the time Alexander the Great conquered the city in 331 BC the ziggurat was falling apart much like it was when Nebuchadnezzar found it 300 years earlier. Alexander, like Nebuchadnezzar before him decided to tear down the ziggurat and rebuild an even grander one. This was natural after all as Alexander's empire was going to dwarf Babylonia.
However, when Alexander died the construction of this massive project stopped and the tiles were left scattered.