Sci-fi book title based on description?

[RitaRogers]

I read this book a long time ago and I can't remember much about it except for I liked it a lot. It's a sci-fi where a man goes to the moon, falls into some sort of hole, and finds an underground world that's controlled by what I think was a plant-type thing. Down there he finds an astronaut that had disappeared years earlier but is now-because of the plant-a vegetable. There's all kinds of weird vegetation in this world. In the end I don't think the guy finds his way out and the plant takes his mind over. All of this is a bit fuzzy though, so I might have gotten a few things wrong. If you have any idea of what this book is that would be great. Thanks!