Accumulations of modern elephant remains have been called "elephants' graveyards", because these sites were erroneously thought to be where old elephants went to die. Similar accumulations of mammoth bones have been found; these are thought to be the result of individuals dying near or in rivers over thousands of years and their bones being accumulated by the water (such as in the Aucilla River in Florida), or animals dying after becoming mired in mud. Some accumulations are thought to be the remains of herds that died at the same time, perhaps due to flooding. Columbian mammoths are occasionally preserved in volcanic deposits such as those in Tocuila, Texcoco, Mexico, where a volcanic lahar mudflow covered at least seven individuals 12,500 years ago. How many mammoths lived at one location at a time is unknown, but the number likely varied by season and lifecycle. Modern elephants can form large herds, sometimes consisting of multiple family groups, and these herds can include thousands of animals migrating together. Mammoths may have formed large herds more often than modern elephants, since animals living in open areas are more likely to do this than those in forested areas.