For almost 100 years 35,000 tonnes of poison gas grenades, abandoned by the fleeing Germans around first world war battlefields, have lain in steel barrels on the seabed less than a kilometre from the Belgian coastline.

A convoy of small boats dumped the cargo into the North Sea over a period of six months in 1919 when unloading it into the world’s oceans was thought to be the best way to protect people from exposure to the toxic material.

The site was soon forgotten, only to be rediscovered under three metres of sand and silt by divers in 1971 during the expansion of the port of Zeebrugge. Building over the barrels was considered given the danger of gas leakage but the costs were thought to be too high. Monitoring was chosen over action.

Yet now, with plans being devised to protect the Belgian coast from rising sea levels through the building of an artificial island off the coast of the seaside resort of Knokke, scientists are starting to explore again whether the construction of a bulwark against the tide could serve two purposes by also keeping the poison gas secure.

“We are just in the process of exploration,” said Jan Seys, a biologist and spokesman for the Flanders Marine Institute. “We are looking at the condition of the seabed. When we start to build the island, can we have a win-win situation?”

Similar toxic dumps exist elsewhere in the world, containing larger amounts of toxic chemicals, but the shallow depth of the site on the Paardenmarkt sandbar, just five to 10 metres below the waterline, is thought to be unique to Belgian waters. Fishing and the dropping of an anchor in the Paardenmarkt is forbidden.

The proximity to Zeebrugge port, one of the busiest in Europe, and major shipping routes, pipelines and gas terminals, is also a factor in the need for a long-term solution.

“We have been monitoring since the 1970s for leakage, and we need to know the state of the ammunition,” Seys said, “but there is a problem in doing seismic tests because natural gas bubbles are created in that area coincidentally and that makes it difficult to see.”

Seys said there was a danger of leakage from the grenades but also in the possibility of impact with a ship given the heavy traffic in the North Sea shipping lanes.

“The liquid inside looks like a viscous liquid, a bit like water saturated with sugar,” he said. “The problem is that this would float on the top of the sea and come in-shore. Or, slowly over time, it could slowly dilute into the sea, increasing the levels of certain metallic elements, such as arsenic. There is also the problem that a ship could lose its way and impact with the ammunition dump, which could be a big problem. We don’t know for sure that there isn’t normal munitions down there.”

Plans for the artificial island of Knokke are part of a 2020-2026 plan for Belgian waters. Further geomorphological surveys of the state of the seabed and the ammunition dump are to be launched over the next year.