The Tree Gnome Stronghold mine (aka. the Grand Tree mine) is located in the cave underneath the Grand Tree within the Tree Gnome Stronghold. In order to gain access to the mine, you have to complete the Grand Tree quest and then twist the stone tile to access the dungeon under the tree. Then push the roots.
The mine is a good alternative to the Mining Guild or Wilderness skeleton mine for Mining coaland perhaps most efficient when player is Mining different kinds of ore.