The wreck of submarine C-3 was only located and identified in 1998. Even if the location had been seen by many people, and still was named by fishermen as "the submarine point", memory had been erased by decades of dictatorship, fear to any reference of Republican war action by Franco censorship.
Many versions were exploited by both sides to hide the sinking of submarine C-3. A shame for the Republican navy, they pretended that C-3 was still active. While Franco's navy made it public that C-3 had changed side: the truth is they were buying 2 u-boots from Mussolini's navy, and named them after 2 submarines sunk. After the war and even after Franco's death, confusion remained and some relatives were still hesitating. about the destiny of their brother or father.
Underwater filming became a real problem, as the C-3 wreck happens to be at 68 meters of depth, 3 miles away from the beaches of Malaga (opposite to the neighborhood of El Palo, popular fishermen quarter). Divers require a mix of gases including hydrogen, therefore a limitation of time and movements that, in combination to the deep and thick waters of the bay of Malaga, made it very difficult to obtain good quality images of the wreck. After several expeditions, only a few images are shown at the end of the documentary, and never shown in a total.
Operation Ursula was the name of the first submarine mission of the Kriegsmarine, Hitler's Navy under the command of Alfred Doenitz. It intended to be so secret that rules of engagement were designed to avoid any mistake or attack on the wrong ships, what might called UK attention and denounce, something Hitler could not afford (while other military actions were happening in land or air, e.g. Guernica bombings, with not so much scandal, the sea was sacred for the British). As a result of these limitations, no ship was sunk by any German torpedo from the 2 u-boots sent to the Spanish Mediterranean coasts from the Northern Sea. In their way back, in late 1936, the Spanish Republican submarine C-3 happened to be the only target for a German torpedo and commander of U-34 did not fail when seeing C-3 in his periscope on the afternoon of December 12th.
Secrecy was revealed by American historian Willard C. Frank, who studied at Washington Congress Library the copies US Government has kept from German secret archives. Later he checked the originals in Germany, at the Friburg military archive, and published it in a specialized magazine in 1980s. Later confirmation came from Antonio Checa, lawyer from Malaga who, while enjoying fishing at his boat, insisted in discovering the origin of some oil drops still coming to the surface from the bottom of the bay. His persistence, together with Willard Frank's data, broke the silence. Their discovery is re-enacted in the film.