Post examples of secrets that were well kept for a very long time (technological, historical etc.)

[Roy]

One of such that I know of is Kyshtym disaster also known as Mayak disaster, at least 200 people died directly due to radiation sickness yet it is an obscure disaster. Russians only officially admitted to it happening and disclosed all the details in 1992 whereas this tragic incident happened in ... 1957.


The disaster was the second-worst nuclear incident (by radioactivity released) after the Chernobyl disaster. It measured as a Level 6 disaster on the International Nuclear Event Scale (INES),[4] making it the third-highest on the INES (which ranks by population impact), behind Chernobyl (evacuated 335,000 people) and the Fukushima Daiichi nuclear disaster (evacuated 154,000 people) which are both Level 7 on the INES. At least twenty-two villages were exposed to radiation from the Kyshtym disaster, with a total population of around 10,000 people evacuated. Some were evacuated after a week, but it took almost two years for evacuations to occur at other sites.[5]

The disaster spread hot particles over more than 52,000 square kilometres (20,000 sq mi), where at least 270,000 people lived.[6] Since Chelyabinsk-40 (later renamed Chelyabinsk-65 until 1994) was not marked on maps, the disaster was named after Kyshtym, the nearest known town.

https://en.wikipedia.org/wiki/Kyshtym_disaster

[Jacques de Imbelloni]

Katyn massacre
The Katyn massacre[a] (https://en.wikipedia.org/wiki/Katyn_massacre#cite_note-1) was a series of mass executions (https://en.wikipedia.org/wiki/Mass_killings_under_Communist_regimes) of nearly 22,000 Polish (https://en.wikipedia.org/wiki/Poles) military officers (https://en.wikipedia.org/wiki/Military_officer) and intelligentsia (https://en.wikipedia.org/wiki/Intelligentsia) carried out by the Soviet Union (https://en.wikipedia.org/wiki/Soviet_Union), specifically the NKVD (https://en.wikipedia.org/wiki/NKVD) ("People's Commissariat for Internal Affairs", the Soviet secret police) in April and May 1940. Though the killings also occurred in the Kalinin (https://en.wikipedia.org/wiki/Tver) and Kharkiv (https://en.wikipedia.org/wiki/Kharkiv) prisons and elsewhere, the massacre is named after the Katyn Forest (https://en.wikipedia.org/wiki/Katyn_Forest), where some of the mass graves (https://en.wikipedia.org/wiki/Mass_grave) were first discovered.
The massacre was initiated in NKVD chief Lavrentiy Beria (https://en.wikipedia.org/wiki/Lavrentiy_Beria)'s proposal to Joseph Stalin (https://en.wikipedia.org/wiki/Joseph_Stalin) to execute all captive members of the Polish officer corps (https://en.wikipedia.org/wiki/Officer_corps), which was approved by the Soviet Politburo (https://en.wikipedia.org/wiki/Soviet_Politburo) led by Stalin. Of the total killed, about 8,000 were officers imprisoned (https://en.wikipedia.org/wiki/Polish_prisoners-of-war_in_the_Soviet_Union_after_1939) during the 1939 Soviet invasion of Poland (https://en.wikipedia.org/wiki/Soviet_invasion_of_Poland), another 6,000 were police officers, and the remaining 8,000 were Polish intelligentsia (https://en.wikipedia.org/wiki/Polish_intelligentsia) the Soviets deemed to be "intelligence agents (https://en.wikipedia.org/wiki/Intelligence_agent), gendarmes (https://en.wikipedia.org/wiki/Gendarmes), landowners, saboteurs, factory owners, lawyers, officials, and priests".[1] (https://en.wikipedia.org/wiki/Katyn_massacre#cite_note-ipn_eng_news_high_katyn_decision-2) The Polish Army officer class was representative of the multi-ethnic Polish state; the murdered included ethnic Poles (https://en.wikipedia.org/wiki/Poles), Ukrainians (https://en.wikipedia.org/wiki/Ukrainians), Belarusians (https://en.wikipedia.org/wiki/Belarusians), and Jews (https://en.wikipedia.org/wiki/History_of_the_Jews_in_Poland) including the chief Rabbi (https://en.wikipedia.org/wiki/Rabbi) of the Polish Army, Baruch Steinberg (https://en.wikipedia.org/wiki/Baruch_Steinberg).[2] (https://en.wikipedia.org/wiki/Katyn_massacre#cite_note-psb-3)
The government of Nazi Germany (https://en.wikipedia.org/wiki/Nazi_Germany) announced the discovery of mass graves in the Katyn Forest in April 1943.[3] (https://en.wikipedia.org/wiki/Katyn_massacre#cite_note-Engel-4) Stalin severed diplomatic relations with the London-based Polish government-in-exile (https://en.wikipedia.org/wiki/Polish_government-in-exile) when it asked for an investigation by the International Committee of the Red Cross (https://en.wikipedia.org/wiki/International_Committee_of_the_Red_Cross). The USSR claimed the Nazis had killed the victims, and it continued to deny responsibility for the massacres until 1990, when it officially acknowledged and condemned the killings by the NKVD, as well as the subsequent cover-up by the Soviet government (https://en.wikipedia.org/wiki/Soviet_government).
An investigation conducted by the office of the prosecutors general of the Soviet Union (https://en.wikipedia.org/wiki/Prosecutors_general_of_the_Soviet_Union) (1990–1991) and the Russian Federation (1991–2004) confirmed Soviet responsibility for the massacres, but refused to classify this action as a war crime (https://en.wikipedia.org/wiki/War_crime) or as an act of mass murder. The investigation was closed on the grounds that the perpetrators were dead, and since the Russian government would not classify the dead as victims of the Great Purge (https://en.wikipedia.org/wiki/Great_Purge), formal posthumous rehabilitation (https://en.wikipedia.org/wiki/Rehabilitation_(Soviet)) was deemed inapplicable. In November 2010, the Russian State Duma (https://en.wikipedia.org/wiki/State_Duma) approved a declaration blaming Stalin and other Soviet officials for ordering the massacre.

[Roy]

Katyn massacre
The Katyn massacre[a] (https://en.wikipedia.org/wiki/Katyn_massacre#cite_note-1) was a series of mass executions (https://en.wikipedia.org/wiki/Mass_killings_under_Communist_regimes) of nearly 22,000 Polish (https://en.wikipedia.org/wiki/Poles) military officers (https://en.wikipedia.org/wiki/Military_officer) and intelligentsia (https://en.wikipedia.org/wiki/Intelligentsia) carried out by the Soviet Union (https://en.wikipedia.org/wiki/Soviet_Union), specifically the NKVD (https://en.wikipedia.org/wiki/NKVD) ("People's Commissariat for Internal Affairs", the Soviet secret police) in April and May 1940. Though the killings also occurred in the Kalinin (https://en.wikipedia.org/wiki/Tver) and Kharkiv (https://en.wikipedia.org/wiki/Kharkiv) prisons and elsewhere, the massacre is named after the Katyn Forest (https://en.wikipedia.org/wiki/Katyn_Forest), where some of the mass graves (https://en.wikipedia.org/wiki/Mass_grave) were first discovered.
The massacre was initiated in NKVD chief Lavrentiy Beria (https://en.wikipedia.org/wiki/Lavrentiy_Beria)'s proposal to Joseph Stalin (https://en.wikipedia.org/wiki/Joseph_Stalin) to execute all captive members of the Polish officer corps (https://en.wikipedia.org/wiki/Officer_corps), which was approved by the Soviet Politburo (https://en.wikipedia.org/wiki/Soviet_Politburo) led by Stalin. Of the total killed, about 8,000 were officers imprisoned (https://en.wikipedia.org/wiki/Polish_prisoners-of-war_in_the_Soviet_Union_after_1939) during the 1939 Soviet invasion of Poland (https://en.wikipedia.org/wiki/Soviet_invasion_of_Poland), another 6,000 were police officers, and the remaining 8,000 were Polish intelligentsia (https://en.wikipedia.org/wiki/Polish_intelligentsia) the Soviets deemed to be "intelligence agents (https://en.wikipedia.org/wiki/Intelligence_agent), gendarmes (https://en.wikipedia.org/wiki/Gendarmes), landowners, saboteurs, factory owners, lawyers, officials, and priests".[1] (https://en.wikipedia.org/wiki/Katyn_massacre#cite_note-ipn_eng_news_high_katyn_decision-2) The Polish Army officer class was representative of the multi-ethnic Polish state; the murdered included ethnic Poles (https://en.wikipedia.org/wiki/Poles), Ukrainians (https://en.wikipedia.org/wiki/Ukrainians), Belarusians (https://en.wikipedia.org/wiki/Belarusians), and Jews (https://en.wikipedia.org/wiki/History_of_the_Jews_in_Poland) including the chief Rabbi (https://en.wikipedia.org/wiki/Rabbi) of the Polish Army, Baruch Steinberg (https://en.wikipedia.org/wiki/Baruch_Steinberg).[2] (https://en.wikipedia.org/wiki/Katyn_massacre#cite_note-psb-3)
The government of Nazi Germany (https://en.wikipedia.org/wiki/Nazi_Germany) announced the discovery of mass graves in the Katyn Forest in April 1943.[3] (https://en.wikipedia.org/wiki/Katyn_massacre#cite_note-Engel-4) Stalin severed diplomatic relations with the London-based Polish government-in-exile (https://en.wikipedia.org/wiki/Polish_government-in-exile) when it asked for an investigation by the International Committee of the Red Cross (https://en.wikipedia.org/wiki/International_Committee_of_the_Red_Cross). The USSR claimed the Nazis had killed the victims, and it continued to deny responsibility for the massacres until 1990, when it officially acknowledged and condemned the killings by the NKVD, as well as the subsequent cover-up by the Soviet government (https://en.wikipedia.org/wiki/Soviet_government).
An investigation conducted by the office of the prosecutors general of the Soviet Union (https://en.wikipedia.org/wiki/Prosecutors_general_of_the_Soviet_Union) (1990–1991) and the Russian Federation (1991–2004) confirmed Soviet responsibility for the massacres, but refused to classify this action as a war crime (https://en.wikipedia.org/wiki/War_crime) or as an act of mass murder. The investigation was closed on the grounds that the perpetrators were dead, and since the Russian government would not classify the dead as victims of the Great Purge (https://en.wikipedia.org/wiki/Great_Purge), formal posthumous rehabilitation (https://en.wikipedia.org/wiki/Rehabilitation_(Soviet)) was deemed inapplicable. In November 2010, the Russian State Duma (https://en.wikipedia.org/wiki/State_Duma) approved a declaration blaming Stalin and other Soviet officials for ordering the massacre.


It's a good example of it but I am looking forward to some that I didn't know of. :)