The secret Super field is only accessible after you reach the 255th floor of the 2nd Mine. After you clear the room of wild animals, a special door will appear that allows you access to a hidden's room. Inside you'll find the Legendary Sword, which you can use to break down the rocks protecting the secret Super field behind the waterfall.
To destroy the large rocks, equip the sword, stand in front of the giant rock, and press the A button. You will swing at the rock and you'll see the stone obstacle fall apart.
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In the mine behind the waterfall/harvest goddess pond
Behind your farm, near the waterfall and mines.
The Special Field is in a cave behind the Waterfall. To get there you must remove the large boulders blocking the path. To do that you must get the Sword at the bottom of the Second Mine.
Behind the waterfall near the mines. You can access it by unlocking the 2nd mine, and getting to the last level. Then, you can either get a sword, or wake up the lady. If you get the sword, you can break the huge rocks that are blocking the path to the cave.
Basically, inside the cave is just a plot of land. You plough and you can plant seeds there. I'm not quite sure for other plants but if you plant yam and after you harvest it, the next day you will find that the plant grew yams AGAIN. And everyday you can harvest yams >.<. But only for that season!! Hope this helps! bx Everything you grow here grows double as fast