Grendelkin cave

Gard, running hard ten paces ahead of me, burst into a chamber. She gathered herself in a sudden leap, flipping neatly in the air, and plummeted from sight. The falling green light of the flare showed me that the tunnel opened into the top of a chamber the size of a small hotel atrium, and if Mouse hadn't stopped first and leaned back against me, I might have slid over the edge before I could stop. As it was, I got a really good look at a drop of at least thirty feet to a wet stone floor.
Gard landed on her feet, turned the momentum into a forward roll, and a shaggy blur the size of an industrial freezer whipped past her, slamming into the wall with a coughing roar and a shudder of impact.
The blonde woman bounced up, kicked off a stone wall, flipped over again, and came down on her feet, ax held high. She'd discarded the flare, leaving it in the center of the floor, and I got my first good look at the place, and at the things in it.
First of all, the chamber, cavern, whatever it was—it was huge. Thirty feet from ceiling to floor, at least thirty feet wide, and it stretched out into the darkness beyond the sharp light cast by the flare. Most of it was natural stone. Some of the surfaces showed signs of being crudely cut with hand-wielded tools. A ledge about two feet wide ran along the edge of the chamber in a C-shape, up near the top. I'd nearly tumbled off the ledge into the cavern. There were stairs cut into the wall below me—if you could call the twelve-inch projections crudely hacked out of the stone every couple of feet a stairway.

Side Jobs, Heorot, Page 136-137