nintendo_dude

First half: Give Healing Seeds (or something like that) to the boy from the Coal Mining Town who's in Circus Village. Once you do he will return to his house in the Coal Mining Town. Visit him and his mother. They will teach you half of the spell.

Second half: In the Coal Mining Town find the house with the pot and break it. The miner will ask you to make him a new one. If you do he will teach you the second half of the spell.

The door is in the Large Mine Underground in an area where it looks like there is a dead end but has a few visable bricks in the wall.

If you use only half of the spell at the entrance the door will appear but you can't enter.