This was the first science fiction book I remember reading as a child, so I was delighted to share it with my seven-year old daughter. It is a Scholastic book and meant for young readers, so it is kid of hard to review on the typical aspects on which I'd judge a book.
I didn't remember many details from when I read it many decades ago, except for one specific situation that comes up on the penultimate chapter. It felt like exposition city as there were many instances of a character launching into exposition, but maybe that is normal for books written for younger readers. Maybe “show, don't tell” is too subtle for younger minds. I do know that every chapter that went on my daughter kept asking “when are they going to meet the character on the cover? Or is that the statue they found? Are they going to be able to talk?” And she really liked the joke about the Wizard of Ooze.
The science is gonzo, but imaginative and fun. It is a variation of the “hollow earth” theme like Pellucidar or Journey to the Center of the Earth. You've got some Matrix-like things, some advanced civilization stuff, and some kind of cliche good civ/bad civ stuff, and some kids who are just kind of along for the ride.
One thing I really enjoyed as I was reading it to my daughter was coming up with different voices for different characters, which my daughter says she likes. Dr. Shaw had a Maine accent. Professor Kincaid sounded like Peter Lorre. Saa sounded a bit like Data. Stan had a deep and manly voice. Gaanu and Krii had a typical villainous voice. The rest kind of all sounded the same.
So is it going to dethrone the Wizard of Oz or the Chronicles of Narnia? No. Is it going to continue to have a place as a nostalgic favorite that led me into reading Tarzan and comic books and enjoying fantasy and science fiction? Absolutely! And I'm very glad I got to enjoy it with my daughter.