Ocean's Silent Threat: Nazi U-Boat 'Mercury Bomb' Discovered Off Norway

Deep beneath the waters west of the Norwegian island of Fedje, a significant piece of German wartime history, the submarine U-864, has rested for over eight decades. Discovered in two fractured sections on the seabed at a depth of approximately 150 metres, the wreck presents a persistent environmental challenge due to its dangerous cargo: a substantial quantity of mercury.

Historical records suggest the U-864 was likely carrying an astonishing 1,867 containers of mercury, destined for Japan, which could equate to between 65 and 67 tonnes if the entire order was loaded. While precise loading documents are lost, it is confirmed that mercury remains within the wreck, posing a long-term environmental risk. Unlike many organic pollutants, mercury does not biologically break down and can persist in contaminated sediments for decades, or even centuries, with the potential for release from soils for hundreds of years, as highlighted by the US environmental agency EPA.

The submarine's final voyage began in February 1945, during the concluding phase of World War II. U-864 embarked on a special mission to transport military and technical equipment, along with specialists, to support Japan’s armaments industry. However, its journey was cut short off Fedje. British intelligence, having decrypted German radio transmissions, enabled the submarine HMS Venturer to track down U-864. On February 9, 1945, Venturer fired a torpedo, hitting and destroying the German submarine. It sank to its current depth, resulting in the loss of all 73 souls on board. This incident stands as the only known historical instance of one submerged submarine successfully sinking another.

For decades, the wreck remained forgotten until clues regarding its unusual cargo resurfaced, prompting investigations off the Norwegian coast. In 2003, the Norwegian naval vessel KNM Tyr successfully located the U-864, confirming its broken state on the seabed. The environmental concern stems from mercury's ability to be converted by micro-organisms into methylmercury, a highly toxic form that bioaccumulates significantly within the food chain, particularly in fish.

Recent examinations of the wreck were conducted in May 2025 by the Norwegian Coastal Administration (Kystverket) using advanced remotely operated underwater vehicles (WROVs), 4K cameras, photogrammetry, and acoustic measurement techniques. These efforts provided new images and facilitated the creation of a three-dimensional model of the wreck and its surrounding seabed, revealing the submarine partially embedded in sediment.

The dangerous cargo poses a significant dilemma for Norway. Many of the presumed 1,867 containers were likely damaged during the sinking. A recovery operation, while potentially removing some mercury, also carries the risk of disturbing contaminated sediments and releasing more pollutants. This was exemplified in 2026, when Kystverket recovered 22 containers, finding that approximately 80% were damaged and leaking. Although about half a tonne of mercury was brought ashore, mercury was also released into the water from the working area during the operation.

Consequently, Norway faces a complex decision: should it attempt a full recovery of the hazardous cargo from the 81-year-old wreck at 150 metres depth, or should the wreck and its contaminated seabed be permanently covered? This question has fueled intense debate for years. Following the 2026 recovery experience, Kystverket reiterated its recommendation to cover the wreck, assessing that a large-scale recovery might ultimately release more pollutants than it would remove. However, covering the wreck would not be a simple final solution, as it would necessitate long-term monitoring.

For the residents of Fedje, the presence of the mercury-laden submarine just a few kilometres off their coast remains a constant concern. While modern technology allows for detailed underwater exploration, the ultimate solution to the U-864's legacy remains elusive. Norwegian authorities have been continuously analyzing fish and shellfish from the region for years, and thankfully, these tests have consistently shown stable or low mercury levels, indicating no immediate cause for undue alarm.