Nuclear fallout shelters are designed to protect for an extended period. Due to the nature of nuclear warfare, such shelters must be completely self-sufficient for long periods. In particular, because of the radioactive contamination of the surrounding area, such a facility must be able to survive for several weeks. In 1959, top-secret construction began in Germany on a government bunker in the Ahr valley. In June 1964, 144 test persons survived for six days in a civilian nuclear bunker. The bunker in Dortmund had been built during the Second World War and had been converted at great expense in the early 1960s into a nuclear-weapon-proof building. However, it would be impossible to build a bunker for millions of German citizens. The Swiss Army built about 7800 nuclear fallout shelters in 1964. In the United States in particular, but also Europe, citizens built private fallout shelters in their front yards on their initiative. This construction was largely kept secret because the owners feared that third parties might take possession of the bunker in the event of a crisis.