Glaciers are simply accumulated snow, packed densely into ice over thousands, even millions, of years. Glaciers serve an important function as ecosystem regulators and water suppliers (they are the largest single source of fresh water), and the heating of the planet has led to major glacier shrinkage around the world in the last decade. Perito Moreno in Argentina, known as one of the few still advancing glaciers on Earth, is a great example of that simple yet stunning ice formation.