Animism was also one of the earliest, enduring and most important belief systems to the Lao-Tai groups, and the traditions and practices which began in Lan Xang have remained a vital part of Lao spirituality. Among the ethnic hill tribes of the Lao Sung and Lao Theung animism was the dominant religion. The Lao Loum believed that ancient mythical serpents known as "ngueak" inhabited major waterways, carving out the surrounding countryside and protecting key points along rivers or other bodies of water. The earliest name for the Mekong River was "Nam Nyai Ngu Luang" or "Great River of the Giant Serpent."