At this time it is only possible to find a maximum of 6 Coal Deposits, and therefore a maximum of only 6 mines at any one time. As a basic resource it is wise to have as many mines in continuous production as possible. Coal can also be made at the Coking House or Recycling Manufactory.
As with other production buildings its output can be temporarily doubled with a Buff and can be upgraded for the duration of the mines current life with Tools, the Tools upgrade is lost once the mine depletes.
Mines can also be replenished with boosts found during randomly after Adventures or purchased with Gems from the Store and prolong the lift of the mine and its upgrade.