An architectural complex that occupies 6,000 square meters on a mountainside in Beijing's Huairou district has long been a mystery for nearby residents.

Called the Beijing College of Mining & Technology, the facility wasn't for students. In fact, it was scientists who started moving in before the buildings were completed. Laboratory equipment delivered to the site was strictly guarded by soldiers.

Now, 61 years after China exploded its first atomic bomb, the Chinese Academy of Sciences has declassified the site - a secret rocket development center and laboratory where China's pioneering scientists once developed satellites and nuclear bombs.