'Big' was a US undertaking within 'Alsos' involving nuclear intelligence-gathering teams to move quickly from Freudenstadt through Horb to Haigerloch in south-western Germany (April/May 1945).
Concerned that French forces might beat the US forces to the Hechingen laboratory of the physicist Werner Heisenberg, Boris T. Pash (a security officer for the 'Manhattan Project' at Los Alamos and, toward the end of the war, the military leader of the 'Alsos' team) extemporised Task Force 'A' as a flying column of US 6th Army Group combat engineers, and this reached Horb within three days and headed for Haigerloch while the French troops were searching for members of the Vichy French government in nearby Sigmaringen. Pash and his engineers, together with Brigadier General Eugene L. Harrison, the 6th Army Group’s intelligence chief, overran Haigerloch on 23 April 1945. In a laboratory within a cave they found a German experimental nuclear reactor whose vessel was empty of uranium and heavy water.
A few drums of heavy water, shipped from Norway after the 'Freshman' and 'Gunnerside' attacks, were later found in the laboratory’s main chamber and a captured German scientist told Pash that the reactor’s uranium cubes had been concealed beneath hay in a nearby barn. Pash later had the empty reactor destroyed.
The task force then proceeded to Hechingen, where its personnel located and detained Erich Bagge, Carl Friedrich von Weizsäcker, Max von Laue, and Karl Wirtz, and then proceeded to Tailfingen, where they arrested Otto Hahn. Heisenberg, who had left Hechingen on 19 April, was captured by Pash and a small force at his home in Urfeld on 3 May.
Pash concluded that the German nuclear programme had been years behind the Manhattan Project and that there was no possibility of Germany being able to mount any form of last-ditch nuclear attack.