Site 6
From Destiny 1 Wiki
Site 6 is a part of Rasputin's Bunker, and is the location where the SIVA Replication Chamber is. It is connected to the Archon's Forge by The Warrens. Once you drop down into the area where the Replication Chamber is, you cannot get back to the patrol area of The Plaguelands, and must return to orbit to get back.