" She had told Erstwhile too much in the past, and thus he knew that occasionally she did go crazy. Sometimes it was when she felt particularly trapped or hopeless, or when the tunnels were unusually dark or stuffy, or when she got stuck in a crawl-through. Sometimes it happened for no obvious reason at all. She would feel a terrible panic tightening her chest and giving her heart a queasy lollop, she would be fighting for breath . . . and then she would be recovering somewhere, shuddering and sick, devastation around her and her fingernails broken from clawing at the rock walls and ceilings. "
― Frances Hardinge , A Face Like Glass