The presence of US nuclear weapons in Europe acted as a deterrent to the Soviet Union during the Cold War and also meant European countries would not need to develop their own.
Various leaked documents have indicated that they use six sites across the continent.
In 2019, a document, written for the Defence and Security Committee of the Nato Parliamentary Assembly, made passing reference to the roughly 150 US nuclear weapons being stored in Europe.
"These bombs are stored at six US and European bases - Kleine Brogel in Belgium, Büchel in Germany, Aviano and Ghedi-Torre in Italy, Volkel in The Netherlands, and Incirlik in Turkey," one line read, according to the Belgian newspaper De Morgen.
The Bellingcat report features screenshots of flashcards indicating that soldiers are taught what to shout to an intruder in the local language.
One card relating to the 701st Munitions Maintenance Squadron shows a phrase to make someone surrender weapons in Flemish, indicating that the security details in it apply to Kleine Brogel airbase, Belgium.
The most revealing information, however, pertained to a "vault status" flashcard that appeared to note which shelters at Volkel contain nuclear weapons.
Five were listed as "hot" and six as "cold".
To further corroborate their story, Bellingcat unearthed a photograph on Facebook posted by someone associated with 703rd MUNSS.
It is a large group photo showing more than 50 individuals wearing US military uniforms posing beside a Dutch army vehicle and in front of a nuclear warhead.
Using geolocating technology and a leaked map of the site, it appears the soldiers are standing in front of vault 532 - which on the flashcard, is listed as "cold".
Dr Jeffrey Lewis, director of the East Asia Nonproliferation Program at the James Martin Centre for Nonproliferation Studies, said it would be highly unlikely for active service members to pose with a live bomb.
Lewis said the flashcard information about the vault being "cold" is likely correct.
The information disclosure is a "flagrant breach" of security practices, he told Bellingcat.
"This is yet one more warning that these weapons are not secure."
The Dutch Ministry of Defence told Bellingcat: "This photo should not have been taken, let alone published."
Hans Kristenssen, director of the Nuclear Information Project at the Federation of American Scientists, said: "There are so many fingerprints that give away where the nuclear weapons are that it serves no military or safety purpose to try to keep it secret. Safety is accomplished by effective security, not secrecy."
But some of the details which soldiers were trying to learn included how to authenticate security badges.
In one screenshot, a flashcard detailed that "VOLKEL" should be spelled without the first L and that 'MUNSS' should be missing an S.
Another card allegedly details where the emblems and flags should appear on the security pass.
A US Air Force Spokesperson told the Telegraph: "The Department of the Air Force is investigating the suitability of information shared via study flashcards."
