coffee's second most important ingredient - water
Water ? how important is it for coffee? The answer is both clear to me and not clear to me. If your water tastes terrible then it will not be good for coffee. Espresso only has two ingridients ? coffee and water, so if one of them tastes bad the end product should taste bad, too.
The ultimate write up on water is called ?the insanely long water FAQ? which not too surprisingly is really, really long. I will not try to reproduce its depth here, but I will summarize some of the important points.
Source: http://www.roaste.com/CoffeeBlogs/wakeknot/coffees-second-most-important-ingredient-water