A gas mine.

The Gas Mine takes the most energy, and produces the least amount of gas, due to it's high value because it is used for fueling fleets. The Gas Mine is used to obtain Gas, which can be used to upgrade buildings, build ships, research new technologies, and other. In addition, mining also takes energy, so you must upgrade your energy production too. Upgrading the Gas Mine increases the production just like the Metal Mine and the Crystal Mine.