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There's no place like Home
After the discussion on the beach, you're welcome back at the Guild! For a while, Diglett examines your footprints, getting extremely nervous and excited that you and your partner are back at the guild. Immediately, there is an uproar and everyone goes out to greet you, and how it's been such a long time. Your partner actually begins to cry, and everyone in the guild (including Diglett) goes to discuss matters with you.
Apparently, you'll have to go to Waterfall Cave in order to see Torkoal at the Hot Spring to find out about the Hidden Land. It hasn't changed much, but as you've grown a lot more levels it should be fairly easy.
After completing the floors, you'll run into the Hot Spring (the partners you bring along don't go into the Hot Spring). Swim over to Torkoal and talk to him.
Torkoal will explain that, in order to get to the Hidden Land, you'll need proof. Like this helps much! Your partner seems to be thinking along the same lines and asks what the proof is anyway. Torkoal gets a small question mark over his head. In order to postpone this answer, he looks around for a bit...but then realizes that he doesn't remember what the proof is. He says if Torkoal remembers, then he would come to the guild to tell them.