I couldn't enjoy this adventure properly. It's inhabited by racist comments, and this distracted me to a point where I considered putting the book away. I know it's an old story, but that doesn't really make it any better. Not to speak of gender issues.
I felt this sotry was a short story stretched to novel lenght. While I like the idea of a hidden prehistoric environment, I found the execution somewhat disappointing. Maybe I expected more detailed action. Also, if you prefer characters who analyze situations and think about stuff, don't read this. It's an adventure story, I know, but: be prepared.
I love the Sherlock Holmes stories, but there is not even a hint in this novel that the author is the same analytic brain who wrote them. Maybe this is what disappointed me most.