How to Become an Expert in Something?

A lot of incredibly intelligent people I look up to are experts in their respective field / domain. They have deep knowledge in a handful of related subjects, while still being relatively knowledgeable about other things, even more than I am at the things I do everyday. I understand that part of it is experience,but many of these people are quite young and I'm not exposed to the sort of problems that would allow me to gain deep knowledge of something and honestly dknt know where to find them half the time. It seems like many of these sort of problems require someone who is already expert. At the moment, I wouldn't even consider myself a good generalist. I have a good superficial knowledge of a large breadth of things and how they work, which is usually enough to satisfy my short term intellectual curiosity, but in practice could theres only a few where I'd be comfortable working with and none I'd be comfortable calling myself an expert in.