What are some activities that one can go deep into as a hobby?

There are a lot of things one can lose oneself into; it’s really a matter of enjoying it. Stamp collecting, coin collecting, reading the Great Books of Literature, choose something you like, and do it.

Collecting brochures is a hobby, although not one you would be able to share with very many people; in fact you may have the only such private collection in Canada.

Caving. You can really go deep into that sport. Sotano de las Golondrinas

10 deepest caves of the world

Anything you do in your free time can be defined as a hobby. Just don’t spank your kids or drop kick your cats.

Also, keep in mind that there is a very fine line between hobby and obsession.

Cosplay. Cosplay is awesome and SOOOOO much fun

There is far more to RPGs than D&D. Take off the training wheels and try Shadowrun… now with a make-your-own editor on Steam.

PC building/repair is also fun, and not terribly expensive if you know how to shop wisely.

fapping to hentai.

Anything that will hold your interest.

Playing a musical instrument.

Deeply..meaning introspective?
Writing your memoirs will is catharic.
ca·thar·tic…..providing psychological relief through the open expression of strong emotions; causing catharsis.

(^^ Don’t you guys talk to each other?)

Genealogy. You can do it for life and trace your ancestors back as far as you are able.