2008 Printz Award winner! 
 I'm going to echo several other reviews here: this is beautifully written, atmospheric, and engrossing, and yet it just didn't do it for me. 
 When Symone's generous uncle offers to whisk her and her mom away to Paris, Sym jumps at the chance--and when her mom's passport goes missing at the last moment, Sym looks forward to exploring Paris with Uncle Victor. But after one night, he reveals that the trip is bigger than Paris. They're going south. Very far south. Antarctica-south, where Sym has dreamed of going. But Uncle Victor isn't just going for the tourism--he's bringing Sym on his expedition to find proof that the Earth is hollow, and that there is an entire society living just under the outer crust. As Victor's obsessive madness becomes more apparent, Sym talks over the situation with someone who understands it all: Titus Oates, the explorer who died here over 90 years ago, whose voice lives in her head and is her confidant, her sounding board, her only friend. 
 Yeah, they're pretty much all crazy. But that's the thing: I'm honestly not sure at what point I was supposed to realize that. At 14 years old, Sym has an imaginary friend, a 90-years-dead explorer, and that's right from the beginning. Uncle Victor is a little shifty straight out of the gate, but Sym's admiration of him blinds her to it--and I can't tell if readers are supposed to be blind to it as well. 
 I'm hard-pressed to find anything specific that I think could be cut, but the book feels overlong. It's not over-written, just too long. The plot drags and there's not a lot of character development (meaning growth and change; the main character is pretty richly drawn, at least). Overall, not my thing, but I can see why it was chosen for the Printz medal.