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Managing legacy munitions in the Baltic Sea: Expedition AL628 investigates suspect sites

It is well known that the Baltic Sea is contaminated with remnants of munitions from past wars. However, many questions remain about the exact locations of these munitions and their condition after decades underwater. Three major projects at national, Baltic Sea and European level are currently working to consolidate existing knowledge and develop technologies to safely identify submerged munitions.

GEOMAR Helmholtz Center for Ocean Research Kiel is contributing to these efforts with three Baltic Sea expeditions. The first mission has now begun: the research vessel ALKOR is testing and further developing state-of-the-art mapping and analysis techniques in German and Danish waters, with a particular focus on the sea areas at the exit of the Flensburg Fjord / Lille Belt, west of Bornholm and the Bay of Lübeck.

Today, they all go into the water: ALBERT, TIFFY, and KÄPT'N BLAUBÄR. The two autonomous underwater vehicles and the small blue remotely operated vehicle from GEOMAR are lowered into the sea to explore the waters southwest of Bornholm. This is no easy task in the strong winds that are almost a constant around the island in the middle of the Baltic Sea between Sweden, Denmark, Poland and Germany.

Good conditions for wind energy, which is why Bornholm is set to become a hub for wind energy. The environmental impact assessments for planned offshore wind farms are currently underway. A key question in this process is: What role do marine munitions play in this, in particular the presence of chemical munitions? What measurements and data are needed to assess the risks and make informed decisions?

MUNI-RISK: Asking the right questions

Developing appropriate guidelines is one of the tasks of the MUNI-RISK project (Mitigation of Risks Due to Submerged Munitions for a Sustainable Development of the Baltic Sea). Since the end of last year, this project, led by the University of Aarhus, has been bringing together experts from a variety of research fields to develop concrete tools and guidelines that will enable the Baltic Sea countries to better assess the risks posed by munitions.

The project is working closely with local authorities, environmental agencies, and experts from industry and business. This also applies to Bornholm, where local stakeholders met with researchers aboard ALKOR on Tuesday, 18 March 2025.

"We want to know what concerns people have about munitions in the sea. What questions do they think science should answer?" explains project leader Dr. Hans Sanderson, an environmental and climate risk expert at Aarhus University. These questions are being asked not only on the Danish island, but also in Estonia and Poland, where wind farms are also planned and dumped munitions could pose a potential risk.

Underwater robots dive into the investigation

Expedition AL628 is the first of three missions to collect data for three major munitions projects: CONMAR, which investigates munitions management in German waters; MUNI-RISK, which focuses on the entire Baltic Sea; and MMinE-SwEEPER, a nine-country European research effort.

In addition to scientific and technological objectives—such as assessing munitions contamination and improving autonomous mapping and analysis methods—the expedition also promotes international cooperation.

"We rely on the exchange of knowledge and data," says chief scientist Prof. Dr. Jens Greinert, marine geologist and munitions expert at GEOMAR. "Where could old munitions be located?"

Looking out over the vast ocean around the ship, the challenge is evident: where to start searching? Even when there are clues, the search remains complex and often time-consuming.

For the area southwest of Bornholm, there were such indications. Jens Greinert explains, "On the other side of the island, chemical munitions were dumped in 1947, and this area here is classified as a relocation area. In the past, when fishermen found remnants of chemical warfare agents in their catch, they were instructed to dispose of them here."

Seeing through the eye of KÄPT'N BLAUBÄR

While ALBERT and TIFFY scan the seabed in the area, KÄPT'N BLAUBÄR's camera allows the research team to monitor the conditions in real time: lots of rocks, some covered in marine growth, but no sign of chemical munitions. Chemical analysis of water samples taken at the same time shows no evidence of explosives. "That is good news," summarizes Greinert.

Next, ALKOR will head to munitions dumping sites in the Bay of Lübeck, where researchers from the Polish institute IOPAN and the German Federal Police will join the expedition.

The final destination will be off the coast of Boltenhagen, where a barge loaded with conventional munitions was sunk after the Second World War. The contents of the barge, which is lying on the seabed, will be cleared by an explosive ordnance disposal company from Rostock in June and July.

The next expedition is planned for October 2025 and will focus on investigating chemical munitions in Polish waters.