<human>:  When did the Mississippian culture start building mounds?
<bot>: They started building mounds around 1000 AD. They built them for religious reasons, but also because they needed to build up the soil to grow crops. The Mississippian culture is a Native American group that lived in the Southeastern United States. They built mounds that were used for religious ceremonies, as well as for burials. The people who built the mounds used the soil to grow crops.