(CNN)Residents of the Swiss village of Mitholz are going to have to find new homes, so authorities can clean up a crumbling ammunition depot filled with tons of explosives.
It could take a decade to remove the weapons stockpile, according to Swiss officials.
The depot was built during World War II inside a mountain in Mitholz, which is about an hour south of Bern.
In 1947, about 7,000 tons of explosives detonated in the underground depot, killing nine people and causing heavy damage to the village.
Part of the facility was rebuilt, and for decades people thought it was safe, according to Federal Department of Defence, Civil Protection and Sport spokeswoman Carolina Bohren.
The Swiss Army later used it to store pharmaceutical supplies, she said.
A risk assessment commissioned in 2018 found that depot is much more dangerous than originally thought and that an estimated 3,500 tons of ammunition is still there.
"It was decided that the munitions have to be cleaned completely from the mountain," Bohren told CNN. "Because the risk of explosion during the cleanup is too high, unfortunately the residents have to move away."
About 170 people live in the village.
The cleanup won't begin until 2031 because of the amount of preparation needed before work can start, Bohren said.
"We announced this now so they can plan their future," she said.
Bohren said a lot of details still need to be worked out, but the government plans to buy residents' homes and property and provide support.
They are working closely with the community and have surveyed residents to find out what they want, she said.
Safety improvements to a road and train line that run through the village will have to be made, so other towns in the region won't be cut off.
At a news conference on Tuesday, Defence Minister Viola Amherd said that the cleanup could cost more than 1 billion Swiss francs ($1.03 billion), Swiss TV reported.
Much of the explosives are buried in unstable rock, which will have to be removed in layers, officials said.
If the stockpile can't be removed safely, authorities may decide to cover the area with rock to bury the ammunition.
Project manager Hanspeter Aelling told reporters that that's not the preferred option because authorities don't want to leave "a toxic gift for descendants," according to Swiss TV.