Before the ship was seized by the Germans in 1940, it operated as a ferry between Calais in France and Dover in England under the name Côte d’Argent. In the spring of 1941, the ship was converted into a minute-laying ship and was given the name Ostmark. Later in the summer, the minute-laying ship Preussen hit a Swedish mine off Öland and the crew was transferred to Ostmark.

In the spring of 1942, the ship was ready for use and carried out exercises against air attacks together with the Luftwaffe. After a period of exercises, mines are loaded on board and the ship heads up to northern Norway to lay them. After its first mission as a minute-laying ship, it goes down to Cuxhaven in Germany and loads 200 mines on board and then sails back to northern Norway where it loads over half of the mines onto Brummer, another minute-laying ship. Both ships then lay their mines and Ostmark then enters the port of Tromsö and loads more mines. Together with the minelayer Ulm, two more minefields are laid. One with submarine mines and one with ship mines, and then the ship enters Tromsø again. Before the ship is sunk in the Kattegat in 1945, a total of 51 minefields will be laid. During Ostmark’s various missions, the ship’s escort varied, but it consisted of various M, S, V, UJ, K, Toch R boats and flying sacs in the form of Focke Wulf, Messerschmitt, Blohm und Voss and Dornier.

In July 1943, Ostmark was in northern Norway on a minelaying mission. While the minelaying was in progress, six Russian bombers attacked along with three fighters. Ostmark and its sacs fired heavy barrage fire, which meant that all the bombs dropped by the Russians missed their targets. During the attack, Ostmark continued laying the minefield that had been started and shortly after the attack, its mission was completed.

The day after the attack, the Russian submarine U-56 surfaced and fired three torpedoes at one of the M-boats that was following the Ostmark. The first torpedo split the M-346 in half and the second torpedo hit the stern, which was completely destroyed. The third submarine torpedo missed its target but almost hit the Ostmark, which dropped two depth charges on the submarine, which then disappeared.

Less than a week later, the Ostmark was again on its way out to lay mines when Russian aircraft attacked. The Germans fired heavy barrage fire, which caused the aircraft to release their bombs prematurely and they then fled the scene. Later that day, the Ostmark carried out its planned minute-by-minute attack.

As the war continued, the Ostmark escaped further air attacks. For example, when it was in port in Stettin in August 1944, when the British carried out bombing raids with nearly 500 Lancaster bombers against the city. Ostmark escaped with a hit from an incendiary bomb and after repairs at the shipyard the ship was active again.

The year 1945 began for Ostmark with a refugee transport from Pillau to Swinemünde. Nearly 1500 refugees, half of whom were children, were shipped over. In Swinemünde mines were loaded on board and went to Copenhagen together with the mine ships Lothringen and Linz and the escort consisting of the two torpedo boats T17 and T20. Minesweepers were M-403 and M-32.

They then remained in Copenhagen for some time before going to Norway. The reason they chose to remain in Copenhagen was that large forces of enemy aircraft were reported in the Kattegat northeast of Elsinore. British aircraft dropped a number of mines and a mine warning was issued throughout the Sound.

Once in Norway, they laid their mines as planned and then laid another minefield before heading towards Kristiansand and anchoring. From here, they then made three attempts to lay another minefield but failed due to bad weather and they unloaded the mines and sailed on 19 April 1945 towards Copenhagen together with Lothringen and M-293.

On 20 April, the weather was very bad with strong winds and high waves. An air raid alert was given at midnight and a 500 m interceptor balloon was sent up and fired barrage fire at enemy aircraft.

Earlier in the evening, three British Halifax MkIII bombers had taken off from Stornoway airport in England on a reconnaissance mission over the Kattegat. At 0005 on 21 April, the German ships were observed and they made a climb and then went on the attack, whereupon a total of 21 bombs were dropped on the ships.

At 0030, Ostmark was hit by two bombs. One in the forecastle where there was also ammunition storage. The explosion was violent. The other hit the deck between the superstructure and the funnel and the ship caught fire. Ammunition nearby exploded, causing the fire to spread quickly. Ostmark had by this time listed to starboard and orders were given to abandon ship. Lifeboats were put into the sea and at 0100, Ostmark capsized and sank shortly afterwards. At the same time, bombs continued to fall on the crew who were in the water. Lothringen and M-293 picked up survivors. Of Ostmark’s crew of 250, 138 survived, of whom seven later died of their injuries. The deceased were buried at the Helsingörs and Västra cemeteries in Copenhagen five days later. At the time of the incident, the Hallandsposten newspaper reported that a large convoy attack of very large scale, “the whole sea was in flames”, was taking place during the night between 20th and 21st April. At midnight, loud explosions were heard from Halmstad out at sea and the pilot’s lookout noted a number of tracer projectiles and a large white flame that was assumed to be an aircraft that crashed into the sea burning.

With the sinking of Ostmark, the flagship of the German minelayers was gone. No more mines were laid during the war.

The wreck of Ostmark lies right on its keel with a slight list to starboard at a depth of about 43 m. The minimum diving depth is about 32 m. The stern is intact with the anti-aircraft gun and a lot of ammunition on deck. Around the skylight are several boxes of grenades and ammunition. The large superstructure in front of the foundation where the funnel once stood has collapsed heavily, partly due to the fierce fighting in April 1945 when the ship was sunk, and partly due to the time the ship has lain on the seabed. In the bow, where one of the British Halifax planes dropped its bombs, the hull side is relatively intact, but up on the foredeck where one of the large guns stood, the deck has completely given way and collapsed. You can clearly see the traces of the battle when you hover above the wreck.