U-23 was laid down at Germaniawerft in Kiel on 11 April 1936 and commissioned on 24 September of the same year. She was a Type IIB — an enlarged version of Germany's earliest U-boat designs, compact and suited to coastal and North Sea operations rather than the open ocean. At 279 tonnes surfaced and 328 tonnes submerged, she was smaller than many fishing vessels, powered by two diesel engines for surface running and electric motors when submerged. Her maximum range was 3,800 nautical miles at eight knots on the surface; underwater, she could manage about 35 nautical miles at four knots before her batteries ran down. Three torpedo tubes at the bow and a complement of twenty-five men completed the picture: a tight, functional machine designed for a specific and brutal purpose.

Over sixteen patrols, U-23 sank seven ships. Numbers like these are easy to recite and easy to process abstractly; they are less easy to hold alongside the reality they represent. On 4 October 1939 — just a month into the war — U-23 torpedoed and sank the British merchant vessel Glen Farg south of Shetland, killing one crew member. Sixteen survivors were rescued and landed at Kirkwall. Over the course of the war, men on seven vessels died or survived because of this one small submarine. Among U-23's ten commanding officers was Otto Kretschmer, who later became the highest-scoring U-boat commander of the war, responsible for the sinking of dozens of ships and the deaths of hundreds of sailors before his own capture in 1941. History tends to attach Kretschmer's name to a long record of tonnage sunk; the men who died in those ships had their own names, their own families, their own reasons for being at sea.

After active service in the Atlantic with the 1st U-boat Flotilla, U-23 shifted to training duties with the 21st U-boat Flotilla from July 1940 until September 1942. Then came a journey unlike almost anything else in naval history. Germany needed submarine capability in the Black Sea to attack Soviet shipping, but there was no direct water route from the Atlantic. The solution was to dismantle the boats entirely, transport the sections by barge up the Elbe and down the Danube, then reassemble them. U-23 was broken apart and shipped to Galați, Romania, where she was put back together at the shipyard and relaunched into the Danube's flow toward the sea. She reached the Black Sea port of Constanța and joined the 30th U-boat Flotilla, operating against Soviet supply lines until September 1944.

By the late summer of 1944, the war in the east had turned decisively against Germany. Romania switched sides on 23 August 1944, and Soviet forces were advancing rapidly. The German and Romanian naval units in the Black Sea faced capture. On 10 September 1944, the crew of U-23 scuttled her — deliberately flooding the boat to sink it — at a position off the coast of Turkey, within Turkish territorial waters. The precise coordinates were not recorded publicly, but the general area was known: somewhere near the coast between İstanbul and the entrance to the Bosphorus. The crew escaped; the boat went down.

For sixty-four years, U-23 lay undisturbed on the Black Sea floor. On 3 February 2008, The Daily Telegraph reported that Turkish marine engineer Selçuk Kolay had located the wreck in approximately 160 feet of water, three miles from the town of Ağva — a small Black Sea coastal village in the Şile district east of İstanbul. The find attracted quiet attention among maritime historians and wreck divers. U-23 is now a time capsule of the Second World War, resting in a sea far from where she was built, far from the North Atlantic where she began her operational life. She is part of the underwater record of a war that changed the world, and a reminder that the consequences of that war — for sailors on all sides, for merchant crews, for the crews of the boats themselves — are still literally lying on the seafloor, waiting to be found.