Limelight

The roof of the cave trembled, then fell in, dumping practicaly a ton of salt dust into the main chamber. Light shone down in streaks. Prof wasn't content with a little hole like we'd made. He'd ripped the top off an entire cavern.

Chapter 41,pg 351,Calamity

I wasn't sure anymore. I tried to force out a reply, but then the ceiling caved in on us.

I stumbled, looking up, but Megan managed to turn the rtich to stop the sudden torrent of stone and dust. Garish sunlight streamed down out of the hole Prof had made, as wide as the entire cavern. I blinked, unaccustomed to the light, and looked at Prof, who ahd stepped out of the way of the downpour and now stood under the lip, in shadow.

"Fire," he said.

Only then did I notice that surrounding the perfect hole about thrity feet above was a squadron of fifty men and women.

Chapter 45, Calamity, pg 382

Context on the depth of the caves.

A three foot by three foot section of ground vaporized, maybe three feet deep. That was very impressive on the old scale of the tensors, but nowhere near what we needed to reach those caverns.

[...]

He released the power with a defiant shout, and it blasted into the floor with such force that it sent me to my knees.

Inches before me, the ground disintegrated into a large hole filled with grains of salt. I watched as the grains siphoned away to reveal a hole a good five feet across. It curved downward, with smooth, glasy sides, traveling through saltstone and then actual rock. The vanishing salt indicated that it opened into something much larger below.

Chapter 41, Calamity pg 346-347.