The Ancient Burial Ground

The Ancient Burial Ground is where the Hollow is kept.

Crypt
It contains the crypt that houses the Hollow.

The Sourcebook
"A realm that exist in infinity, outside if time and space, where the crypt that houses the Hollow is located."

Innocents Lost
Neena and Hogan go to The Ancient Burial Ground to get some dirt from the ground. They use this in both the spell to revive the Source and in Neena's spell to share the Source's power.