IndustrialCraft2, often stylized as IC2, is a mod for Minecraft based around industrial processes and is one of the most venerable mods still in use in Feed The Beast today.

IC2 introduces numerous features across a variety of areas in Minecraft, including resource processing, mining, agriculture and the player's armory itself. It accomplishes this through the use of a proprietary energy system of Energy Units, or EU, and includes an array of generators and blocks to provide and store this energy and a much larger stable of machinery and tools to use this energy in their own ways.

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Official Wiki (Tutorial/First Steps)
Official Wiki (Recipes and Resources)
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