Kurdish heritage is rooted in one of the world's oldest cultures. The earliest known evidence of a unified and distinct culture (and, possibly, ethnicity) of the inhabitants of the Kurdish mountains dates back to the Halaf culture of 5400 - 6000 B.C. This was followed which was a foreign introduction from Mesopotamia. Kurds consider themselves Indo-European descendants of the Guti, Hattians, Kassites, Mitanni, Mannai, Urartu, and Mushku groups. All of these peoples shared a common identity and spoke one language or closely related languages or dialects. According to the Encyclopedia Kurdistanica, Kurds are the descendants of all those who settled in Kurdistan historically, not of any one particular group. Those ancestors include the Guti (Kurti), Mede, Mard, Carduchi (Gordyaei), Adiabene, Zila and Khaldi. This heritage has been subject to injustices, neglect and repression, or has been eclipsed by other cultures. Important components of the Kurds' original cultural heritage have disappeared or have been destroyed. There are numerous examples of how valuable or irreplaceable Kurdish physical heritage has been endangered or destroyed, such as the threat posed by the Illusi Dam in Turkey, where the ancient Kurdish city of Hasankeyf could soon be covered by water.