First, you have to go Underground. Then Read this:
One of the main attractions of the Underground is the mining minigame. To mine, the player must find a spot where the wall 'pings' by touching the screen and looking for sparkles. These spots will appear randomly in the Underground, usually in clusters, and can be located by looking for sparkles marked on the radar screen. They appear more frequently near buried spheres, and so one way players can ensure they always have a lot of mining spots available to them is to make a "sphere garden" by burying many spheres in the same area.
The game itself is similar to Battleship or Minesweeper. At the start, the cave wall is shown, represented by a grid full of squares. There are three depths of wall: the tan soil, which can be broken through in one hit, the darker soil, which needs two hits to be broken through, and the large rocks, which each need three hits to be broken through. The player digs into the wall with a pick, which chips away small areas of rock, and a hammer, which breaks a larger area of the rock. Each spot will hide one to four items, including spheres, fossils, Revives, Heart Scales, shards, evolutionary stones, plates, and a few held items. Iron pieces are also in the wall, which cannot be dug out and prevent further mining upon discovery (that is, when they are uncovered on a direct hit, no other spaces are revealed until the next hit). Upon hitting a piece of iron a small 'ping' will be heard. Each hit will bring the wall closer to collapsing; if the wall collapses before everything is dug up, the game ends and the player is rewarded with what they dug up before the wall collapsed.