http://redlib.nowherejezfoltodf4jiyl6r56jnzintap5vyjlia7fkirfsnfizflqd.onion/r/hacking
Through CTF you learn literally everything there is about the digital world, in a rather intense but exciting way. Almost all the creators/finders of major exploits have dabbled in CTF in some way/form, and almost all of them have helped solve real-world issues.