My dad once said to me, "You give a lot of books three stars." I do.
An almost perfect ending to a decent MG series.
Really good tension in the middle. So good I had to jump ahead in a few places to see if everything was going to be OK or not.*
Spoilers under the cut.
OK but aliens? Seriously? I was on board until I got to the aliens.
Also if I were ever to reread this series I would just read The City of Ember and The Diamond of Darkhold. The other two books are pretty unnecessary after you've read them once.
Also why are the diamonds glass? Storing a thousand glass objects inside a mountain in earthquake country for 250+ years doesn't sound very practical to me. One word: plastics.
*I actually wish there had been a few more disasters. It felt like everything was leading up to doom but the other shoe never dropped.