The pristine turquoise waters of Lake Geneva may appear serene, but lurking below are piles of bombs, cartridges and possibly even chemical weapons discarded decades ago.

Long believed to be safely slumbering beneath thick layers of protective sediment, the munitions at the bottom of the biggest lake in the Alps have raised fresh safety and environmental concerns.

"We believe there are bombs and shells, and probably rifle ammunition," Jacques Martelain, the Geneva canton's head geologist, told AFP.

Some also fear there might also be phosgene bombs -- deadly chemical weapons -- sitting on the bottom of the lake, he said.

For the first time ever, Swiss authorities will soon start mapping the piles of munitions in the lake to determine what kind of explosive debris is there, how much, and whether it should be removed.

Switzerland is a famously neutral country that did not fight in the two world wars, but its long-held position is one of well-armed neutrality.

Between World War I and the mid-1960s, thousands of tonnes of munitions, from artillery to grenades and detonators, were sunk in lakes across the nation.

Following two explosions in storage depots, one of them inside a mountain, the army started to get rid of surplus post-World War II stockpiles by using underwater dumps.

Over the decades, the Swiss army is estimated to have dumped more than 8,000 tonnes of munitions in the Thun, Lucerne and Brienz lakes.

Authorities studied those stockpiles carefully and decided around a decade ago that it was safer to leave them where they were, resting at significant depths and covered with thick layers of sediment.

They estimated that there was little danger in leaving them untouched, while removing them risked shifting the sediment, releasing pollutants and causing significant damage to the aquatic ecosystem.

- Sonar tracking -

However, experts have warned that the situation is different in Lake Geneva, where a private armaments company, Hispano-Suiza, dumped excess munitions right up until the 1960s.

The company, which no longer exists, had a number of arms factories in Geneva, but cantonal authorities don't know why it used the lake as a dumping ground.

The Swiss defence ministry estimated in the early 2000s that between 150 and 1,000 tonnes of munitions had been sunk, but did not pinpoint their exact location or detail the specific weaponry.

In the coming weeks, authorities will begin testing out tracking techniques in the lake, which provides Geneva with around 80 percent of its drinking water.

"We will detect these metal masses from boats using immerged sonar equipment," Martelain said.

One of the main concerns is that the munition stocks are believed to be resting at far shallower depths than in other lakes.

The crescent-shaped freshwater lake -- 73 kilometres (45 miles) long and 14 kilometres across at its widest -- hits a maximum depth of 310 metres (1,017 feet) in the middle, making it the deepest lake in Switzerland.

But the munitions search will focus on the far shallower end near the city of Geneva itself, an area popular with swimmers where the lake only reaches depths of 50 to 100 metres.

- Souvenir hunters -

The Geneva authorities long thought the sediment that covered the weapons provided protection, as in other lakes.

They also thought they were deeper, but in 2019 the French environmental organisation Odysseus 3.1 discovered several disembowelled ammunition crates at a depth of just 50 metres -- uncovered by sediment.

Salima Moyard, a diver and former Geneva regional parliamentarian, has spent years fighting for the cantonal authorities to map the weaponry in the lake, insisting that a "complete clean-up" would be needed eventually.

She worries that amateur divers could go hunting for the weapons, a potentially risky endeavour.

"There could be individuals who go looking for the ammunition to display it on their mantlepiece," she told AFP.

"That would be really serious: serious for the people themselves, for their neighbours, for the environment."

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