A mine is a building you can ask Willy Builder to make for 16,500 coins. It is the only way that you will be able to get a Dragonache egg. You have to hire Diggerlings to work in it. If you give a Candary a gas mask it can work in the mines too. If you build one you might want to get a Profitamole to eat the mine workings (the big piles of soil) that sometimes come out. There are many things you can get from the mine that make it worth while other than the dragonache egg.