<human>:  Who were the Mound builders?
<bot>: The Mound Builders were a group of Native Americans who lived in the Mississippi River Valley. They built many large earthen mounds, including the famous Cahokia Mounds. The Mound Builders were a sophisticated society, and they had a complex system of trade and exchange. They were also known for their art and architecture, and they had a sophisticated system of writing.