From Asimov to Zelazny
When I was in high school, I read every single science fiction book in the Clearfield Public Library. Probably 250 books altogether.
I don't think I had a big plan, I was mostly looking for something to do. What I discovered, though, was that domain knowledge, edge to edge knowledge of a field, was incredibly valuable. It helped me understand where the edges were, and it gave me the confidence to be selective, to develop a taxonomy, to see what was going on.
As the deluge of information grows and choices continue to widen (there's no way I could even attempt to cover science fiction from scratch today, for example), it's easy to forget the benefits of acquiring this sort of (mostly) complete understanding in a field. I'm not even sure it matters which field you pick.
Expertise is a posture as much as it is a volume of knowledge.
Reading every single trade journal, for example, or understanding the marketing, engineering and sales of your field--there are countless ways to go deep instead of merely paying lip service to the current flavor of the moment.