Ok, this is really only for the lower levels...
At the Fargodeep Mine, on the outside by the mincarts under the shaft thing, a chest spawns like every 1-5minutes.
the chest usually contains Malachite, Light Leather, Copper Ore, red Apples, Linen, potions, peacebloom and silver leaf, rough stones, water, etc.
This ii helpful for those lower lvls who are doing quests there, want to build some professions, and/or linen farmers.