The Plains Indians believed in underwater spirits who controlled all animals and plants.  Above the sky, they believed their was an upper world ruled by the Thunderbirds.  The Thunderbirds were the most powerful of the spirits.
 
The Indians believed that spirits could control their health.  Plains Indians honored and greatly respected the spirits they believed in.  The Indians honored the spirits with the creation of their medicine bundles, Medicine Pipes, and religious ceremonies.

Below is a photo of a group of Lakota Indians (a Plains Indians tribe)  preparing to perform the Sun Dance, a religious ceremony, in which the Indians ask the sun to give them strength.