According to this article at dailymail.co.uk and this article at news.com.au a declassified document "APO 696" (purportedly in the U.S. National Archives) includes testimony that indicates a possible nuclear explosion in Germany in early October 1944.
Above newspaper articles say that the "APO 696" document contains the following statement by a German test pilot Hans Zinsser:
In early October 1944 I flew away 12-15 km from a nuclear test station near Ludwigslust (south of Lübeck). A cloud shaped like a mushroom with turbulent, billowing sections (at about 7000 metres) stood, without any seeming connections over the spot where the explosion took place. Strong electrical disturbances and the impossibility to continue radio communication as by lighting turned up.
The Daily Mail article continues that Hans Zinsser
estimated the cloud stretching for 6.5miles and described further 'strange colourings' followed by a blast wave which translated into a 'strong pull on the stick' - meaning his cockpit controls.
I have not found online access to the "APO 696" document in the U.S. National Archives so I cannot confirm whether it states what is claimed in these articles.
However both articles also refer to the explosion to have been seen by a reporter named Luigi Romersa. According to this article in Spiegel Online (dated March 14, 2005) and the Wikipedia article about him, Mr. Romersa was a former war reporter for the Milanese newspaper Corriere della Sera (engl. "Evening Courier"). The Spiegel article states that:
For years Romersa, a Roman who is now 87, has been telling the story of how he visited Hitler in October 1944 and then was flown to an island in the Baltic Sea. Romersa says that he was taken to a dugout where he witnessed an explosion that produced a bright light, and that men wearing protective suits then drove him away from the site, telling him that what he had witnessed was a "fission bomb."
The news.com.au article relates above report to an incident involving a German treasure hunter by name Bernd Thälmann who, as it is told, had located a lump of uranium in the soil near Oranienburg (a German article about this is here at berliner-kurier.de, and an English one can be found here at independent.co.uk).
A problem I have with determining the truthfulness of these claims is that each story seems to me refer to a different location for the explosion. Consequently we have reports that would indicate WW2-era nuclear activity in
- Ludwigslust (south of Lübeck)
- an island in the Baltic Sea
- near Oranienburg (seems to be near Berlin?)
Is it plausible that there was a nuclear explosion in Germany during WW2?