Imp mine north of Northrop:
(Torch needed. One jump, or a magic floating bridge optional. Also get a pickaxe from north-eastern hut in the Fura snowfield, accessible when it snows, to mine the sparkly spots.)
1. Start on B1
2. Take the Left elevator to B3. This will cause the other elevator to move to B3 as well (the sign on this floor says so).
3. Exit south and press the switch on the south-west wall next to the grate (this will open up a path towards the end of this guide).
4. Back to the elevator, exit north and find and take the ladder up till you return to B1.
5. Walk into the Right elevator shaft on B1 (you will fall down to B2).
6. Exit north and find the ladder down to B3. You will find the mine key here.
7. Take the ladders back up to B1. (Clear the north-western room on B2 on the way.)
8. Take the north-eastern ladder down to B3 (you can unlock the B2 door with the mine key on your way down) and take the left elevator to B4. Somehow, the right elevator will go to B2 now. (Do not use the left elevator again, or the right elevator will go elsewhere. So we have to take the ladders from here on out.)
9. Exit north, taking the doors for treasure, etc if you want. The north-western corner room will warp you to the north-eastern room with a treasure chest. Two doors have no floor. The north-east one will drop you to B5. You might as well take the stairs down to B6 while you're here if you want to 100% the dungeon. Afterwards, take the stairs back to the B4 Left elevator and exit north again.
10. Take the south-western-most door with no floor (need mine key). You will drop to a room with a chest on B5. There's also a north-western room with orichalcum to mine. Fall down from the elevator shaft to B6.
11. Clear out B6 (use a torch) while you're here, and fight your way to the south-western ladder. Climb the ladders back up to B1.
12. Take the south-eastern ladder down to B2. Now you can take the Right elevator to B5. Kill the imps here in the big room. Destroy the magic circle in the next room to finish the imp quest.
13. To finish up, take the Right elevator up to B3. There is a door requiring the mine key to the north. Take it and go around to the south-east corner (this is what the previous switch opened up). Use your magic door to walk north back to the elevator area, then take the eastern door to find a ladder up to B2. You can exit through a one-way door.
14. Take the ladder up to B1, and you may have to drop down the south-western room to 100% the dungeon.