Tourists enjoying the picture postcard views of Lakes Lucerne, Thun or Lake Neuchatel might be surprised to learn what lies beneath those pristine alpine waters.

For years the Swiss military used the lakes as dumping grounds for old munition, believing it could be disposed of safely there.

In Lake Lucerne alone there are an estimated 3,300 tonnes of munition, and 4,500 tonnes in the waters of Neuchatel, which the Swiss air force used for bombing practice until 2021.

Some munitions are at depths of 150 to 220 metres, but others in Lake Neuchatel are just six or seven metres below the surface.

Now, the Swiss defence department is offering 50,000 francs (£45,000) in prize money for the best idea to get it out.

The best three ideas for a safe and environmental solution to retrieve the munition will share the prize pot – but the salvage operation is expected to cost billions.

The fact that so many rounds were dumped in Swiss lakes – Brienz being another of them – has been known about for decades, though people have asked questions about safety more recently.

Retired Swiss geologist Marcos Buser, who advised the government on this topic, wrote a research paper ten years ago warning of the dangers of the dumps.

The munition poses two risks, he said. First, despite the fact it is underwater, there is still a risk of explosion, because in many cases “the army did not remove the fuses before dumping the munition”.

Then there’s water and soil contamination – there is a real chance that highly toxic TNT could pollute the lake water and the sediment.

The Swiss government acknowledges that factors including poor visibility, magnetic iron and individual ammunition weights “represent major challenges for environmentally friendly ammunition recovery”.

An assessment of possible recovery techniques in 2005 showed that all proposed solutions for ammunition recovery posed severe risks for the sensitive ecosystems of the lakes.

It’s not the first time Switzerland’s military has appeared somewhat negligent with its munitions.

The alpine village of Mitholz suffered a massive blast in 1947, when 3,000 tonnes of ammunition the army had stored in the mountain overlooking the village exploded.

Nine people were killed, and the village was destroyed. The blast was even heard 160 kilometres (100 miles) away in Zurich.

Three years ago, the military revealed that 3,500 tonnes of unexploded ammunition which still remained buried in the mountain was not safe after all, and said it would be removed.

For Mitholz’s residents that meant leaving home for up to a decade while the clean-up operation took place.

There have also been scandals about neutral Switzerland’s Cold War defence strategy of mining its bridges and tunnels against an invasion. Some bridges had to be quickly demined because modern heavy goods vehicles risked triggering an explosion.

In 2001, 11 people died in the Gotthard Tunnel, one of Europe’s key north to south transport routes, when a fire broke out following a collision between two lorries.

Large quantities of explosives – not involved in the explosion – were still stored in a depot close to the tunnel mouth and after fire officers successfully put the fire out, the army arrived, bomb disposal equipment to hand.

And this week, the army revealed that reports from civilians finding unexploded ordinance out in the Swiss countryside increased by 12% last year (on 2022).

Even on the glaciers, now receding amid the impacts of climate change, the melting ice is revealing spent and live ammunition left over from high mountain training which took place decades ago.

It’s a legacy of Switzerland’s ‘armed neutrality’ defence strategy – maintaining a large militia army (all Swiss men are required to do military service) which trains its troops almost entirely inside what is a densely populated country.

The task to remove munition from Switzerland’s lakes is expected be long and complicated. But first, someone needs to come up with a workable plan of how exactly to get them out.

While some complain that the army should have thought of that while dumping, for decades the advice from geologists to the military was that the practice was safe.

The hunt is on for solutions. Following the Swiss defence department’s appeal, the public can submit their ideas until February next year when they will be anonymously judged by a panel of experts.

The three winners will be announced next April.

The government said: “It is not planned to implement the submitted entries immediately, but they could serve as the basis for further clarifications or for launching research projects.”

Mr Buser suggests turning to the UK, Norway, or Denmark for advice given their experience in dealing with wartime wrecks containing unexploded weapons.

So will he offer any ideas? “No, I’m too old now…but if they need any advice I will be pleased to give it.”