The aspects of the Umbra beyond the Horizon & far from Earth. The Shard Realms exist here.
Nephandi Labyrinths are thought to be in the Deep Umbra.
1st ed core pg. 180 “Mages are more likely to travel the Deep Umbra, though. The laws of reality are completely annulled in the Deep Umbra. In such a place, a mage is free to explore the limits of her potential without interference, restriction or fear of Paradox. The Deep Umbra is accessible to many mages, but this does not mean they understand it.”
Pg. 16 of Mage Storytellers Handbook gives a good understanding of the relationship between physical space and the Deep Umbra. The Gauntlet is extremely thin in space. It’s easy to slip between physical space & the Deep Umbra (if you are Awakened or have any kind of spirit/magic ability). Beyond Mars Shallowings exist.
In first 2 editions of Mage Tradition mages could breathe in Etherspace while Technocrats couldn’t. In Infinite Tapestry pg. 108 it’s an Arete roll.
Pg. 15 of Book of Madness has the Fringe Realms. These are in the deepest part of the Deep Umbra and home to the fearful entities that the Outsider Nephandi serve. The Fringe Realms are not mentioned again.