Freshwater is a more precious commodity than many people in the developed world recognize. More than 97 percent of Earth’s water is salty, unfit for drinking. Of the remainder, more than two-thirds is frozen in glaciers and icecaps, leaving just 1 percent of all the water on our planet fresh. About one one-hundredth of that 1 percent resides in rivers and other waterways; the rest is beneath the earth's surface. Pressure on this invaluable resource is growing. In the last sentence, the phrase “this invaluable resource” most probably refers to _____.