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. 
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. 
Realm of Ghyran
- 1: The Realmgate Wars: Balance of Power Rifts between Realms, p52-53
- 2: Warhammer Community: the Lore of Gnawholes