Residents of the Swiss village of Mitholz have been asked to relocate for at least 10 years to allow authorities to clear a 3,500-tonne stockpile of munitions sitting in an underground depot.
A risk assessment of the underground depot carried out over the previous two years said it presents "unacceptable" risks based on current safety regulations.
The depot, which used to contain 7,000 tonnes of explosives, exploded in 1947, killing nine people.
The government says the recent study turned up greater risks than did earlier inspections in 1949 and 1986, which determined that explosions could still take place at the site, but that damage would likely be limited to the facility itself. Back then, authorities deemed any cleanup as too risky -- mainly for geological reasons.
The ammunition depot was created as part of a Swiss military strategy that held if Switzerland, which was officially neutral during World War II, were attacked, its soldiers would hole up in mountain hideaways and benefit from a network of underground munitions stashes scattered across the Alpine country.
The operation to remove the explosives from the depot is not projected to start until 2030 to allow time to secure railway lines and national roads in the area of the facility.
"For security reasons, according to current knowledge, the residents of Mitholz must also move away at the latest during the evacuation for more than 10 years, depending on the course of the evacuation," the government said in a statement.
Local authorities and residents were consulted about the project, and have given their approval, it added.
Overall, the entire operation is expected to cost between CHF 500-900 million (€463-835 million).