The National Park Service offers several cave tours to visitors. Some notable features of the cave, such as Grand Avenue, and Frozen Niagara, can be seen on lighted tours raging from one to six hours in length. The story of human beings in relation to Mammoth Cave spans six thousand years. Several sets of Native American remains have been recovered from Mammoth Cave, or nearby caves in the region, in both the 19th and 20th centuries. Most mummies found represent examples of intentional burial, with ample evidence of pre-Columbian funerary practice.