Bill Bryson is very good at explaining not only what we know, but how we came to know it, which often makes for a better story.

We know amazingly little about what happens beneath our feet. It is fairly remarkable to think that Ford has been building cars and baseball has been playing World Series for longer than we have known that the Earth has a core. And of course the idea that the continents move about on the surface like lily pads has been common wisdom for much less than a generation. “Strange as it may seem,” wrote Richard Feynman, “we understand the distribution of matter in the interior of the Sun far better than we understand the interior of the Earth.”

...Until slightly under a century ago, what the best-informed scientific minds knew about Earth's interior was not much more than what a coal miner knew – namely, that you could dig down through soil for a distance and then you'd hit rock and that was about it.

...By the 1960s scientists had grown sufficiently frustrated by how little they understood of the Earth's interior that they decided to do something about it. Specifically, they got the idea to drill through the ocean floor (the continental crust was too thick) to the Moho discontinuity and to extract a piece of the Earth's mantle for examination at leisure.... The project became known, all but inevitably, as the Mohole and it was pretty well disastrous.... Every attempt ended in failure. The deepest they penetrated was only about 600 feet. The Mohole became known as the No Hole.