From a long article about the movie:
Marshall grew up in Northern England; he can trace the inspiration for The Descent back to harrowing school trips into local mineshafts. “I don’t think they’d be taking kids down mines these days,” he says, chuckling. “Deep underground, the instructor told us all to turn our torches off and, for the first time, experience pitch-black: zero light, absolutely nothing,” he recalls.
“We turn off the lights, and the guy says, ‘By the way, don’t wander off, because there’s a 200-foot drop over there.’”
BTW:
Filming in caves was considered too dangerous and time-consuming, so the subterranean setting was built at Pinewood Studios