Most of Cambodia's northern border with Thailand is formed by the escarpment of the Dângrêk Mountains. A natural pass cuts through the mountains between O Smach and Chong Chom. This pass has been used since ancient times to travel between the plains of lower Cambodia and the Khorat Plateau. Beginning with the Cambodian Civil War and lasting until the surrender of the last remaining Khmer Rouge who had taken refuge in Anlong Veng, the Khmer Rouge controlled the area and generated income by illegally trading Cambodian timber across the border in Thailand. After the final surrender of the Khmer Rouge in 1999, the region surrounding the O Smach pass once again became stable and safe for travelers and, in 2003, the Cambodian and Thai governments opened an international border crossing. The border crossing suffered intermittent closures again between 2008 and 2011 during the Cambodian–Thai border dispute.