Old sci-fi book about the center of the Earth. Children's? YA? Maybe?

I saw this book while in the waiting room of the hospital when I was probably in fourth grade, which would've been around 1999 or 2000. I went heavy on reading it and they let me take it home. I was a pretty serious reader even back then so I dunno how much of a kid's book it was, I remember it being pretty thick and having that old style minimalist cover, kind of leathery, with the title engraved into it.

I feel like the title was "deep trouble" but searching for that doesn't find me anything.

Anyway, I think it started off with characters scuba diving, but eventually they ended up in tunnels beneath the earth, inhabited by lizard people. These tunnels went all the way to the core of the earth.

I don't really remember much of the plot except for a few specific scenes. I remember somewhere in the middle of the book they drove around in a burrowing vehicle that I imagined looking like a submarine. At the end of the book I think they were blown out of the underground by a volcano or a geyser, and near the end they found pictures of dinosaurs that had been taken by the lizard people.

I've been looking for this for years now, so it'd be awesome if somebody knows what it is.