If Russia had an equivalent to our Area 51, then Kapustin Yar would most likely be it. Although it was constructed in 1946 as a missile test site, it wouldn’t be until 1983 before the Soviet government even acknowledged it existence. Once it was confirmed to exist it was discovered that it was also the location where Laika, the dog that became the first living thing to orbit the earth, was launched from.
Strangely, like with Area 51, people who have claimed to work at the facility have claimed the same outlandish things like crashed alien craft and alien autopsies being constructed in the mainly underground base. What has fueled such conspiracy theories is the fact that it is known that the Kapustin Yar base has a large underground element to it, we just don’t know how large and for what purpose.