And here I am... as soon as I put the logbook in my pocket and made few steps, the ground collapsed and I fell into a narrow, slippery tunnel! I was sliding for a while before I fell out. I think I'm now a few floors below the surface... 

Luckily the landing was soft, there's a carpet of spongy moss on the ground. I can also put my LED lantern aside, there are beautiful shining crystals around that give me enough light. It's amazing!

I made some photos of them.