Perhaps it was simply because William's body was still so near, but the reminder that some<em>thing</em>would very likely soon do its best to kill her left Isabel strangely unaffected.  She'd known she was moving towards danger the moment she'd left her home, and this was far more important than a single life.  She set off after the Doctor as he moved ahead again, slightly hesitant at first in the dim light but slowly gaining confidence.<br/><br/>Soon the opening to the caves loomed not far ahead of them.  It was a small opening for the first two or three yards, one that would force even Isabel to duck a little, but it opened up quickly into a larger space, as she recalled from her furtive, forbidden explorations of the caves as a child.  Beyond lay more than enough room for these Cybermen to work their terrible mischief on the countryside, unless and until they were stopped.  She was still not sure she was up to the task of stopping them, but she did not allow herself to dwell on that thought.<br/><br/>As they drew nearer, the noise of the vibrations became a gentle hum, as the Doctor had predicted, a steady sound that seemed to come from everywhere and nowhere, like the humming of a huge, hidden swarm of insects.  And inside the opening she could also barely detect a faint light, a glow that seemed to suggest a much brighter light that was mostly hidden from view.  Something gave her the sense of movement inside as well, though that could just as easily have been imagination, and she looked to the Doctor for guidance as she slowed her pace.