|Required for||Units: Ironclad|
Coal is the first new strategic resource to become available after a long time with only Horses and Iron. Coal is used for two very specific things: Ironclad ships, which are a transition between ships with sails and modern engine-based ships; and the construction of Factories.
Of these, the second is much more important, firstly because Factories are a major Production booster for a city, and what's more, they're required for a civilization to adopt Ideology early. Since each Factory consumes 1 Coal, you will need at least 3 Coal to be among the first to adopt an Ideology, and reap the free bonus Tenets.
Coal is accessed with a Mine. Coal is perhaps the rarest of all strategic resources, along with Uranium - Coal is found in locations of difficult access, like Jungles, and its sources are much less than Horses or Iron. It is often the case that you will realize that you have practically no Coal within easy access, or that the sources within your reach are much closer to another empire.
You will have to expand additionally to get it, and you should be prepared for that come the Industrial Era, or you will be forced to trade for Coal for the rest of the game. Luckily, once you build a Factory, there doesn't seem to be any negative effect if you start running negative Coal resource.
Its second use, the Ironclad ships, are only a temporary drain - when you replace them with the later Destroyer you will liberate the Coal they used for more Factories.
Coal is ancient plant matter that has decomposed and compressed over the millennia into a highly combustible black compound. Found in substantial quantities across virtually the entire planet, coal has been one of the most important sources of energy to mankind throughout history. Coal is not a clean fuel and its extraction and burning is a major source of pollution around the world. Scientists are frantically looking for the elusive "clean coal" technology, but that remains as of yet undiscovered. Remember that coal is a strategic resource, and thus it is consumed as you construct the associated units and buildings.