There are two mines in Harvest Moon: Back to Nature. The Waterfall Mine is open all year and you can find the ores you need to upgrade your tools there. The other mine is open only during the winter of this videogame when the pond on the mountain freezes over and you can find several other rare ores that sell for more and can be used to make things like jewelry or the maker machines.
If you buy the basket from the Supermarket you’ll be able to take that into the mine and it will allow you to gather way more ore per trip.
Both mines have ten floors. A trick to find the ladder to the next floor is to dig around the area on the opposite side of the room. For example if the ladder back up is in the bottom left corner of the room dig around the upper right corner to find the ladder down to the next floor.
The ladder will also never be in any of the squares on the perimeter of the room so don’t bother digging there. Dig at least one square in from the perimeter to find the ladder.
In this videogame you’ll also find a pond on the bottom floor of the Winter mine where you can catch a King Fish.
With both mines you’ll dig up better items the deeper you go. You can also dig up bags of 10g, especially in the Waterfall Mine, and you’ll dig up a Power Berry randomly in each mine too.