And when you have a civilization that was around for so long, if there were people there as early as 7,000 BCE, we're talking about it was there for thousands of years, but all of a sudden it starts to decline. There's evidence of less and less trade going on, less and less sophistication, and then it ends. And it's one of the mysteries of history, of archeology today. Why did this Indus Valley civilization end? Some of the older theories was maybe it was a foreign invasion, maybe some of the ancestors of the modern Indians invaded, and maybe they assimilated it somehow. More current theories don't think that was the case. They think it might be some form of climate change that some of the important rivers dried up, made the agriculture much harder.