OK... I finished this book. I also really enjoyed it. I do agree with many of the comments.
I wondered near the end... would it not have been possible to blast in the caldera to relive the pressure at the point where it appears to have gone the past time.
I mean they lowered the level of a lake by creating a waterfall with improvised explosive. At the end they had plenty of black powder from the brig and from Ayrtons Tabor island stores.
As far as the animals... they did build a corral to house them and raise them... chickens for eggs and yes to eat, onagers which they used like mules, musmons for their fur.
I personally have no problem eating animals... even today that is the staple of a good portion of the world.
Since some people have read what the "mystery" is... I'll spoiler some comments about that:
I think this was a great book club selection... thanks to HarryT for suggesting it.