Tibesti, also Tibesti Mountains, mountainous region of the central Sahara, in northern Chad, extending into northeastern Niger and southern Libya. The mountains are of volcanic formation and rise abruptly above the surrounding plains. The highest peak, Emi Koussi, has an elevation of 3,415 m (11,204 ft). The presence here of deep-cut stream beds and ancient rock carvings depicting hippopotamuses and elephants indicate the existence of a more humid climate in the past.