The Underground Sanctuary is the newest and main branch of the Sanctuary Network, hidden at an undisclosed location beneath the surface of the Earth. It is the largest and most modern Sanctuary ever built, and is believed to hold enclosures for all of the Abnormals currently under the protection of the Sanctuary Network as well as having more than enough occupancy space remaining for the inclusion of many more future residents across the world that will be found and given sanctuary.
Dr. Helen Magnus realized that her concept for the Network would not work into the 24th century, so she made preparations to change its nature.
After Prue Halliwell's funeral, the new underground headquarters was already up and running.
Magnus revealed its existence to Will Zimmerman (at some point within the week following his quitting from SCIU), Typhuss James Kira and Helena Kyle after Prue Halliwell's funeral , noting there was a lot of work to do.
Current known residents living in this Sanctuary are all of the Abnormals from the Old City Sanctuary as well as all of the Abnormals from the 12 Sanctuaries Magnus had closed in 8 months between mid 2386 and early 2387; of those, specifically: Brisbane Sanctuary, Cape Town Sanctuary, Dallas Sanctuary, and Tokyo Sanctuary.
A notable individual residing at this Sanctuary is Magnus's daughter, Ashley. During Magnus's time in the past, she had interfered with the EM shield to let Ashley pass through the barrier; redirecting her energy signature to land at this location for her to rematerialize. Based on the fact that Ashley hasn't been noticeably seen topside by any personnel connected to the Sanctuary project since the Cabal super abnormal attacks in early 2371, it may indicate that she has been living underground exclusively for the past 3 years.