The mine is where you can mine ores to sell and, materials to upgrade your tools.
Ores can be mined with the Hammer, and to access the next Mine level you have to use your Hoe on the Mine ground until you find the ladder.

After you've unlock the mountain you will need to build the second bridge to have acess to the Mine using:
[5x Softwood Lumber + 600G]
To build the Mine you will need:
[10x Softwood Lumber + 500G]

The mine have 100 levels, to reach the next level, you will need to dig it up to find the hole to the next level.
The mine map is divided in a 3x3 grade or 4x4 grade, the square that have ores will never have the hole to the next floor.
In every 5 levels a checpoint is made in the mine entrance.

Making money with the minning is verry easy.

After you get the Material you will need to Melt it to get a higher price, you can Melt 3 Materials into a plate in Gus Hut.

After you get the Ore you will need to lapidate it to get a higher price, you can lapidate 1 Ore in Gus Hut.