Once you get an explorers kit, you can go underground. Once there, just tap the lower screen until you see sparkles on the walls and/or ground. Then walk up to them and press A. You should be able to dig for items there. You can find a range of useful items such as Heart Scales and Spheres that you can trade for valuable items with the people you find in the underground.
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Spheres in Pokemon diamond are found underground. Go underground using the explorer kit, touch the touch screen and look at the some shinning things(this means there is a sphere or treasure), if you're near the shinning things, touch the touch screen and you will see were they are located (they are still shinning things but you can see them in the touch screen), then go in front of them then press A.
Pokemon diamond
you get a digger drill from an old man in a house next to the PC or you can buy it from hikers for about size 11 spheres
you cant be a Pokemon in Pokemon diamond