It was built in a rock cellar in Hohenzollerischen Lande Haigerloch early in 1945 as part of the German nuclear program during World War II.

The suggestion to use the Hohenzollerische Lande for this probably came from the head of the Physics Division in the Reichsforschungsrat, Walther Gerlach, who had studied at the University of Tübingen and was a professor there in the late 1920s, making him familiar with the area.

At the same time, the Kaiser Wilhelm Institute for Chemistry, was relocated to nearby Tailfingen (now part of Albstadt-Tailfingen), with prominent scientists such as Otto Hahn and Max von Laue involved in the move.

Wirtz was setting up the largest German reactor test to date in the still-intact Dahlem institute bunker when the Red Army advanced to within 80 km of Berlin.

[4] As early as July 29, 1944, the accidentally discovered potato and beer cellar of the Haigerloch Schwanenwirt was rented for 100 Reichsmark per month as the new location of the Berlin research reactor.

[6] In the narrow Eyachtal valley, it was driven into the mountain under the Schlosskirche (castle church) there and protected against bomb attacks by a 20–30 m thick layer of rock made of shell limestone.

After a night-time journey on an icy freeway, the convoy stopped the following day about 240 kilometers south of Berlin in Thuringia Stadtilm, where Diebner's working group had been relocated the previous summer.

Very annoyed at the change of plan, Wirtz contacted Heisenberg in Hechingen, who immediately set off for Stadtilm together with von Weizsäcker and arrived there three days later after an adventurous journey by bicycle, train and car.

Trucks were again procured and on February 23, 1945, the physicist Erich Bagge set off from Haigerloch with a new convoy to collect the materials from their storage site in Stadtilm.

[12] A total of 664 cubes made of natural uranium with an edge length of five centimetres and a weight of 2.4 kilograms each were attached to this lid using 78 aluminum wires.

During the following experiment, the heavy water, which was stored in three large tanks at the end of the tunnel, was also filled into the inner reactor vessel through the chimney.

[1] If, contrary to all expectations, the plant had gotten out of control, the cadmium piece, which acted as a neutron absorber, would have been thrown into the reactor through the chimney, thus interrupting the chain reaction.

Von Weizsäcker had also pointed out its usability as a weapon early on, as well as the fact that a new fissile element - later known as plutonium - would have to be created in uranium reactors.

[19] In principle, the Haigerloch tests could have confirmed these assumptions, but the scientists were also aware that many years of extensive research would have been necessary to develop operational weapons.

[22] In a final attempt to make the reactor critical after all, Heisenberg wanted to transport the remaining heavy water and uranium that was left in Stadtilm to Haigerloch.

The aim of the US special unit Alsos, founded in 1943 as part of the Manhattan Project under General Leslie R. Groves, was to expose and secure the German nuclear research facilities and to capture the leading scientists.

The aim was not only to advance Germany's own nuclear weapons program, but also to prevent the Soviet Union and the other later occupying forces from using the knowledge.

The military head of the mission was Lieutenant Colonel Boris Pash, the scientific team was led by the Dutch-born physicist Samuel Goudsmit.

In order to get ahead of the French troops, Groves and Pash considered attacking the facility from the air with paratroopers or destroying it with bombing raids.

There, Goudsmit learned that the nuclear research facilities of the uranium project had been relocated to Haigerloch near Hechingen and to Stadtilm in the future Soviet occupation zone.

They managed to arrive there about three weeks before the Soviet forces, but Diebner had already fled with his employees and materials towards Munich in the future American occupation zone.

However, after hours of interrogation, Wirtz and von Weizsäcker were coaxed into revealing the hiding places with the false promise that they would be allowed to resume their experiments after the war under the protection of the Allies.

Von Weizsäcker had hidden the scientific documents, including the top-secret Nuclear physics research reports, in a cesspit behind his house in Hechingen.

[34] A French task force led by the physicist Yves Rocard, which arrived in Hechingen shortly after the US troops in search of the facility, found only a piece of uranium from a laboratory the size of a sugar cube.

There, in August 1945, they learned of the atomic bombs dropped on Hiroshima and Nagasaki and thus also of the progress made by the Americans in nuclear technology and its consequences.

[25] The two-part German television film End of Innocence from 1991 documents the development of the uranium project from the discovery of nuclear fission in 1938 to the experiments in Haigerloch and the subsequent internment of the scientists in 1945.

[40] The play Copenhagen by Michael Frayn from 1998 is about a fictional meeting between Heisenberg and Niels Bohr and his wife Margarete at an unspecified point in time after the end of the war.

At the end of the first act, Heisenberg reflects on the work on the Haigerloch research reactor, the lack of safety measures and the endeavor to achieve criticality for the first time.

[41] A real meeting between the two men had taken place during the war in Copenhagen in 1941, but it is not clear from the documents that still exist today what was said at the time and specifically how it was meant and interpreted.

In a flashback, we follow one of the two protagonists as he uncovers the German nuclear program in Heidelberg, Hechingen and Haigerloch as a fictitious part of the Alsos mission together with Goudsmit and Pash.