I already done all quests thats i can find on map, so i don't know what to do else and decide to dig a tonnel in mountain.  When i dig this tonnel big mob appear near me, so i start thinking how to spawn more mobs for farm and decide to expand my tonnel. After a while, when i dig out pretty big space this appears in my cave

So i enter this portal and it teleport me to totally closed area ! 

i walk there a little all doors still closed and idk what else i can do there. There is only 1 NPC and he just greetings me, but this still pretty cool.

What else i can do here ? 

You can go north one world tile, then use the world map to move. The crystal also sends you to a random location.