Much of the northern and central Durand line is quite mountainous, where crossing the border is often only practical in the numerous passes through the mountains. Border crossings are very common, especially among Pashtuns who cross the border to meet relatives or to work. The movement of people crossing the border has largely been unchecked or uncontrolled, although passports and visas are at times checked at official crossings. In June 2011 the United States installed a biometric system at the border crossing near Spin Boldak aimed at improving the security situation and blocking the infiltration of insurgents into southern Afghanistan.