In October 2025, RV ALKOR, with 12 scientists and technicians from GEOMAR and IOPAN, worked in the western Baltic Sea. During this cruise, MMinE-SwEEPER joined forces with the MUNI-RISK and CONMAR projects to continue mapping munition pollution. As the first working area, the Bornholm Basin, with dumped chemical munitions, was targeted. To survey large areas in a short time, the MBES of RV ALKOR was used, and the 6000 m deep-diving AUV ABYSS was deployed at the same time to map the seafloor with SSS. Some data from previous cruises were used to plan remapping the northern part of the indicated munition dumping ground.
The data clearly show individual KC250 bombs, most of them open, broken, and empty. Some had a ‘strange’ orange seafloor cover next to some of them. Whether this cover represents spilled and partially converted chemicals or specific bacterial mats cannot be determined yet. A few other bombs showed light brown, yellowish lumps, which are believed to be encrusted mustard gas remains, as reported previously from this area.
In addition to seafloor mapping, a spatially wide CTD grid collected water samples, which were analyzed on board for explosive compounds. No significant increase in TNT, DNB, or RDX was detected. At almost 100 m depth, the Bornholm dump site is rather deep but shows a dense grid of fishing activities (pelagic net fishing with bottom-touching weight), including actual bottom trawling.
In contrast, the Little Belt area is shallower and revealed, during AL 628 in March 2025, one large pile of bombs on the seafloor. Using AUV ABYSS, we mapped large parts of the indicated dump. Unfortunately, a strong pycnocline prevented the same data quality as in the Bornholm Basin. Nevertheless, we were able to identify another cluster of KC250 bombs, densely scattered in a 100 by 70 m area.
Finally, we also worked in German waters in Lübeck Bay. Here, the targets of a ship-based SAS system were F103 rocket warheads, which we expected to be present based on MBES data. The SAS clearly shows the large (1.4 by 0.7 m) barrel-shaped objects. In some parts, they are completely corroded, and only the large metal rings at each side of the objects still exist, creating large circular shadows in the SAS images.
All this work is part of work packages 3, 5, 8, and 10, which deal with mapping technologies for munition objects on the seafloor. The data will be used in WP5 for annotation and later AI training purposes.
