Gnawholes are uncanny burrows that tunnel through the fabric of reality, bridging the void between the Mortal Realms
Unlike other races, the Skaven are able to use Gnawholes to move between realms without the use of a Realmgate with the added safeguard that they are lethal for any but the skaven to use due to their dangerous, chaotic energies. 
In the Age of Myth, the Verminlords shared the knowledge of how to create Gnawholes with the Skaven.
During the Age of Chaos, Clans Shrykt dug an vast gnawhole in the Realm of Aqshy and plunged into its depths - Neither the Great Clan or the gnawhole have ever been seen again. 
Even before Sigmar unleashed his forces into the Mortal Realms, operatives of Clans Eshin were able to spy on the God-King’s throne room, returning to report to their mysterious master through a hidden gnawhole.
Each of the rival Great Clans have their own secret and arcane means of creating a gnawhole. Clans Pestilens create virulent plagues that can chew through reality itself whereas Clans Skryre use technology - gigantic reality-piercing drills. Like many Skaven technologies and magic, gnawholes are somewhat unpredictable causing many skaven to perish when a gnawhole accidentally burrows into a volcano or ocean. 
- Deepengnaw: A shadowy gnawhole where the Shadow Council meet, it is guarded by vicious curses that no mortal being could endure. It is constantly fluctuating in size and appearance.[3a]
Realm of Ghyran
- 1: The Realmgate Wars: Balance of Power Rifts between Realms, p52-53
- 2: Warhammer Community: the Lore of Gnawholes
- 3: Battletome: Skaven (2019)
- 3a: The Horned Rat, pg. 8-9