The second type of dam known to have been built was an earth dam called Nimrod's Dam in Mesopotamia around 2000 BC. Earth dams are massive dams similar to gravity dams except they are made of soil. The dam is made watertight, with a core wall and filled with an impervious center usually made of clays. Nimrod's dam was built north of Baghdad across the Tigris and was used to prevent erosion and reduce the threat of flooding. The intention was to divert the flow in the river and help irrigate the crops. The dam was built of earth and wood, so it is difficult to be certain of the exact characteristics of the dam.