But people of the Indus Valley were trading with Mesopotamians as early as 3500 BCE. We also know that it was the largest of the ancient civilizations. Archaeologists have discovered more than 1500 sites. So what do we know about this civilization? Let's go to the Thought Bubble. Everything we know about the Indus Valley Civilization comes from archaeology, because while they did use written language, we don't know how to read it, and no Rosetta Stone has thus far appeared to help us learn it. I meant the other Rosetta Stone Thought Bubble.