TNT in the mussels: a 1917 U-boat off Romo is still dosing the seabed around it

The informative part of the UC-30 work is which samples came back dirty. Traces of TNT were in the seabed and the water column, and also in the mussels and starfish living on the hull ^[8]. The barrier holding the charge in is a mine casing, and casings fail slowly, which is why Andresen expects the signal to grow rather than fade as more of the explosive filling is exposed ^[9]. That is the opposite slope to most legacy contamination, where the source term decays and monitoring can be tapered.

The single-hull inventory is worth stating plainly. UC-30 went down in April 1917 with 18 mines and six torpedoes aboard ^[2], which is 24 explosive charges in one boat ^[2], and it sat unlocated until a Danish diver found it in 2016 ^[4]. Andresen's 2024 report for the Danish Environmental Protection Agency counted 10,127 wrecks in Danish waters, roughly 15 percent of them from the two war periods ^[12][13]. That is on the order of 1,500 hulls of the relevant vintage ^[1]. The report does not say how many of those carry ordnance, so treat 1,500 as the pool to triage, not a queue of remediation jobs.

The commissioning question matters more than the archaeology. The agency asked for the report to work out how Denmark can comply with the EU Water Framework Directive ^[14], which is a question about the chemical status of a water body, not about heritage protection. Andresen says she does not know whether the agency has taken the work further since publication ^[15]. A named, quantified, worsening source inside a water body is the sort of thing that eventually needs an owner and a budget line.

Access is its own constraint on ever knowing more. Civilian diving on UC-30 is prohibited, so the sampling was done by navy divers working to instructions, who also shot the video and stills used to judge how exposed the mines are and what condition the wreck is in ^[6][7]. Sampling at the scale of a national wreck register means buying military dive time or robotic equivalents, repeatedly, because the answer changes as the steel goes.

And the number that would actually price the problem is still missing. Andresen frames it herself: pollution is happening, the open questions are how significant the effect is and how far from the wreck it reaches, with her expectation that the plume is fairly localised ^[9]. Without a distance figure there is no defensible buffer radius, and without a radius nobody can compare leaving a wreck alone against lifting it. Germany has committed funds to recovering some Baltic wrecks and the ammunition the Allies dumped at the end of the war ^[16], and Andresen notes that neutralising retrieved explosives is very costly ^[17]. Costly is not a number either.