Ningxia has rich historical heritages. The Shuidonggou Site was one of the first Paleolithic sites excavated in China. Ningxia is also known as the Chinese Great Wall Museum, hosting relics of the Great Wall built from the Warring States period to the Ming Dynasty. The Rock Art in the Helan Mountain and the Damaidi Rock Art are precious treasures of ethnic art and culture. The Grottoes on the Xumi Mountain, first built in the late Northern Wei Dynasty, is among the top ten grottoes in China. The Imperial Mausoleum of Western Xia is known as the Oriental Pyramid.