GENEVA (AP) — Swiss authorities say a 3,500-ton stockpile of munitions sitting in an underground depot in the Bernese Alps since World War II must be cleared for safety reasons, advancing toward a giant project that could cost billions and require the evacuation of local residents from their homes for a decade — though likely not before 2030.

The government said Monday that the Defense Ministry has been asked to plan out needed safety and evacuation measures by the autumn of 2022 at the site in Mitholz, which is home to the stockpile that once held around 7,000 tons of munitions. Part of it exploded in 1947, killing nine people.

Authorities say the Mitholz depot, following an analysis that began two years ago, continues to present “unacceptable” risks based on current safety regulations. Among the operations required will be securing transportation networks in the region of central Switzerland.

Local authorities and residents were consulted about the project, and have given their approval, the government said Monday.

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.