There are two different mines you can dig in. The Spring Mine is next to the Goddess' pond and the Lake Mine is on an island in the lake on Mother's Hill. The Lake Mine is only normally accessible during the winter when the lake's water freezes so you can walk to the island. If you have the Teleport Stone you can go there in any season.
A floor of the mine consists of a staircase back up to the surface, rocks, dirt, and a hidden staircase down to the next floor that you have to find. The only tools you'll need are your Hammer and Hoe. To get down to the next floor of the mine you have to find the exit hidden in the dirt, which you can uncover with your Hoe. The exit will be in the general area of the entrance, but you won't find it hidden on the 8 squares that directly touch the entrance staircase. Likewise you won't find an exit along any of the room walls. The exits can be hidden in the ground under rocks, which you can smash open to reveal goodies to sell.
Each mine is 255 floors deep. To reach the far depths of the mines you're going to need to use both save slots of your game. Just going around smashing and digging is going to tire you out too quickly. Before you go down a staircase, save your game in slot 1. When you reach the new floor save again in slot 2. From there smash and dig until you find the exit. Instead of going down to the next floor, keep track of where the exit staircase was and then load your game from slot 2. Then just walk directly to the spot were the down staircase is and dig it up! Before you go down to the new level remember to save in slot 1 and save again in slot 2 once you reach the new floor. Sometimes you'll find a room that doesn't have and exit down to the next floor. These "dead-ends" can be a pain in the butt, especially since there's no way to go up 1 floor. If you remember to save your game in slot 1 before you go down, you can simply reload your slot 1 game if you find a dead-end room. You will eventually get tired, so don't forget to bring a rucksack full of goodies to eat and recharge your stamina.
It's a good idea to only bring the Hammer and Hoe tools with you when you plan on going deep into the mine. Any tools you find hidden in the mines won't appear unless you have adaquate space in your rucksack to store them. The same goes with the Goddess and Kappa gems, but they need empty slots available in the Items section of your rucksack.
The best type of food to bring with you is Elli Leaves. When you eat the grass you'll
fully recharge your energy! You have to make Elli Leaves by using your Kitchen and it's
not a simple recipe.
Step 1 - Bodigizer and Turbojolt Potions
You can buy these at the Hospital or you can make them. If you choose to buy the potions, the Bodigizer costs 500 G and the Turbojolt costs 1,000 G. If you want to make them you're going to need to collect a few ingredients:
The Red Magic Grass only grow in the Fall season. When you plant Magic Grass Seeds
from Won, there's a small chance that one of the flowers will sprout as a red-colored
one instead of a normal blue-colored one. Black Grass can be found by digging in the
mines, Orange Grass on the beach during the Summer, and White Grass behind the Church
or Gotz's house during the Winter. If you give 10 items to the Goddess, she'll usually
give you some White Grass in any season. See, I told you it's not easy ;)
Step 2 - Bodigizer XL and Turbojolt XL Potions
You can actually skip over Step 1 if your game has the Bodigizer XL and Turbojolt XL Potions for sale at the Clinic. The Boldigizer XL costs 1,000 G and the Turbojolt XL is 2,000 G. If you don't want to buy them (or can't) then you can make them instead.
The Blue Grass is found in the Spring and Summer around the Goddess' Pond and Gotz's
House. The Green Grass appears in the Summer and Fall by the Lake and the Goddess'
Pond. The XL potions will appear for sale in the Clinic after you ship 50 Blue Grass
and 50 Green Grass.
Step 3 - Burn a lot of food!
Now that you have your potions, it's time to make the other 6 ingredients required for Elli Leaves. You're going to need 6 types of "burnt" food, which are created when you screw up a cooking recipe. The cheapest way of making burnt food is to use Weeds or even Chicken Feed with one of your cooking utensils. You'll need to use 1 "food" with the Knife, Frying Pan, Pot, Mixer, Whisk, and Rolling Pin. The burnt food that comes from the Oven is the same kind as if you use the Rolling Pin.
Step 4 - Combine it all together
The only utensils you won't be using is the Whisk, Mixer, and Rolling Pin. For seasoning you're going to use the Sugar, Salt, Vinegar, Soy Sauce, and Miso. Once you're finished you'll have an edible item that you can use to help you reach the lowest floors of the mines.