"And the human body tends to dislike things entering it from outside." It would be no use going inside, just to end up making them ill. Still, if he could teach biology that way the students would probably be more likely to listen. Maybe he should ask Vicki, if he ever saw her again, if she was taught that way.

"I went once," he said, to John question. "Not officially though. Me and my brother came across some caves once and went down them. By the time we came out at the other end it was dark - a blackout - and we had to walk for miles to get home." But he smiled at the memory - it had been a adventure and he hadn't really regretted it, despite the scolding they'd had when they got home.

He leaned out from the ladder to look down and up. "We're over halfway there," he guessed. "I can see why not many people would want to come down here."