Water is a critical ingredient in bourbon making, or at least the right kind of water is. As luck would have it, Kentucky's natural spring water is ideal for the creation of its native whiskey.
Allow me to go on a brief geological tangent. Limestone is abundant in Kentucky. This type of sedimentary rock was formed 460 million to 330 million year ago when warm, shallow seas covered what is presently Kentucky. The subterranean limestone now acts as a natural filter for Kentucky's spring water.
As is important to the bourbon industry, when groundwater flows through the limestone, iron salts are removed from it and calcium and magnesium are added. As a result, the water can react more favorably with the yeast during the fermentation process, as it provides an ideal environment for yeast to live and multiply. Without such suitable water, i.e. water that contains iron or other minerals, the water will react with the chemicals in the barrel wood and turn the bourbon bitter and black, and sometimes bright green. Again, the right water is a critical factor in making drinkable whiskey.
I guess ideal bourbon-making water is truly millions of years in the making.