context: The main part of the system was built in the 1950s and 1960s, the construction was completely completed in the 1980s. The length of the line, along with all branches, is slightly less than 25 km. Depth of laying - from 50 meters in the center to 210 meters in the area of ​​the NIBO "Science"
question: When was the secret Moscow underground transport facility Metro-2 built?
answers.text: The main part of the system was built in the 1950s and 1960s