This was a common theme in science fiction at the end of the 19th century. The Hollow Earth Theory. I found it a rather good book but very dated.

I found the book dated and the science awful even for its time. The author forgot about inertia and g-forces. Such acceleration would have killed them. As the old saying goes. Those who can't do, teach. And John Campbell was a great teacher.

I found it rather a very nonsensical line of garbage. Trying to be a stupid text adventure with a CEO trying to find his girl? Grabbing at straws this story does.

This story blows chunks. Religious preaching in science fiction form. At least with Burroughs we get a better way to get to mars.