Area 52 was a facility in River Song's World which resembled the Great Pyramid of Giza from the Doctor's World.
It was controlled by a group of people who had noticed what had happened to time, led by Amy Pond. They worked endlessly to find a way to make time move again and remove the intertwining time periods from each other. Over a hundred Silents were kept in this facility, as well as their agent, Madame Kovarian.
The base was secured by River Song through the use of her hallucinogenic lipstick on President Kennedy and pointing a gun at Cleopatra. A device built by River was located on top of the pyramid. It was used to call everyone in the universe throughout time, asking for help to save the Doctor from death. (TV: The Wedding of River Song)