(32.8 ft) wide was created along the entire length of the inner German border. An adjoining "protective strip" (Schutzstreifen) 500 m (1,640 ft) wide was placed under tight control. A "restricted zone" (Sperrzone) a further 5 km (3.1 mile) wide was created in which only those holding a special permit could live or work. Trees and brush were cut down along the border to clear lines of sight for the guards and to eliminate cover for would-be crossers. Houses adjoining the border were torn down, bridges were closed and barbed-wire fencing was put up in many places. Farmers were permitted to work their fields along the border only in daylight hours and under the watch of armed guards, who were authorised to use weapons if their orders were not obeyed. Border communities on both sides suffered acute disruption. Farms, coal mines and even houses were split in two by the sudden closure of the border. More than 8,300 East German civilians living along the border were forcibly resettled in a programme codenamed "Operation Vermin" (Aktion Ungeziefer). Another 3,000 residents, realising they were about to be expelled from their homes, fled to the West. The seal around the country was expanded in July 1962 when the GDR declared its entire Baltic coast a border zone subject to closures and restrictions. The border between East and West Berlin was also significantly tightened, although not fully closed; East Germans were still able to cross into West Berlin, which then became the main route by which East Germans migrated to the West. Between 1949 and the building of the Berlin Wall in 1961, an estimated 3.5 million East Germans – a sixth of the entire population – emigrated to the West, most via Berlin. 1967–89: the "Modern Frontier" The GDR decided to upgrade the fortifications in the late 1960s to establish a "modern frontier" that would be far more difficult to cross. Barbed-wire fences were replaced with harder-to-climb expanded metal barriers; directional anti-personnel mines and anti-vehicle ditches blocked the movement of people and vehicles; tripwires and electric signals helped guards to detect escapees; all-weather patrol roads enabled rapid access to any point along the border; and wooden guard towers were replaced with prefabricated concrete towers and observation bunkers. Construction of the new border system started in September 1967. Nearly of new fencing was built, usually further back from the geographical line than the old