The Tunnels

The tunnels are a subterranean network of passages that run under The Magnus Institute and connect the building to the old site of Mill Bank Prison. The tunnels were designed by Robert Smirke and feature his twisting and senseless architectural style.
For several years prior to his death, Jurgen Leitner lived in the tunnels, on the run and living in a clandestine fashion, so that the agents of The Entities would not be able to locate him. This is because the tunnels provide strong protections from the Powers and those that inhabit it are somewhat immune to them. He also used the tunnels to trap Not-Sasha between two walls when he manipulated their shape using the powers of a special book he had in his possession. 
After The Mass Ritual, the tunnels becomes a relatively safe place for survivors to dwell as they are shielded from The Eye. After Melanie King and Georgie Barker began pulling more survivors from the domains of the Powers, the tunnels became the centre of a cult that worshiped them.