Janus' lab was a hidden lab in the lower levels of the tallest tower on the East pier of Atlantis where the Ancient scientist Janus would conduct his research without the Lantean Council knowing about it (as there would be potentially severe consequences if his research was discovered). There, he kept a device that was essential to the activation of the Attero device.
The lab was discovered by the Atlantis expedition in 2008 by Dr. Rodney McKay and Dr. Daniel Jackson. They discovered it in an abandoned part of the city that had suffered water damage during the expedition's first year in the city. The room is located beyond a wall at the end of a hallway. To access it, three lights that reside along a wall in the hallway must be touched in sequence to create a sort of harmonic resonance. Once activated in the proper sequence, one must only step through the Ancient wall, rather quickly, to emerge in Janus' lab. However, Dr. McKay made some modifications allowing the wall to remain open constantly. It was in this lab that both Dr. McKay and Dr. Jackson were kidnapped by the Vanir and the Attero device control key was stolen. (SGA: "First Contact", "The Lost Tribe")
Following the departure of Jackson from Atlantis, the expedition continued their work in the lab. Months later, McKay noted that they had recovered "tons" of Ancient technology from the lab and were still in the process of cataloging it all. While in the lab with Dr. Jennifer Keller, she inadvertently activated a Long-range communication stone, which then allowed her to later switch bodies with Neeva Casol. (SGA: "Identity")