The Pegasus Alpha Site is an outpost set up by the Atlantis expedition as early as their first year in the Pegasus Galaxy. The original Alpha Site was abandoned after being discovered, and tagged, by the Wraith and the expedition had to search for a replacement. (SGA: "The Gift")
A new Alpha Site was set up in the second year and was used as a temporary prison (and research) facility for the Wraith known as Michael Kenmore. This base was likely abandoned after Michael escaped and returned to the Wraith. (SGA: "Michael")