www.undergroundworldnews.com A new map of Earth's groundwater supply shows where on the planet water is locked up and "hidden" underground. The map — the first of its kind — provides a visual representation of Earth's groundwater resources and estimates that the planet's total groundwater supply stands at approximately 5.5 million cubic miles (about 23 million cubic kilometers). Groundwater is the source of the world's second-largest collection of freshwater, according to the National Ground Water Association. (The planet's primary source of fresh water comes from glaciers and ice caps.) Groundwater is collected from rainfall that seeps underground into aquifers and reservoirs beneath the land surface, according to the U.S. Environmental Protection Agency (EPA). [Earth in the Balance: 7 Crucial Tipping Points] Groundwater is important for energy and food security, human health and healthy ecosystems, but it's also a resource that is at risk from overuse and human pollution, researchers said in a new study published online Nov. 16 in the journal Nature Geoscience. The scientists obtained measurements of tritium, a radioactive version of the hydrogen molecule, and water flow models that used properties of water and its interaction with different types of rock to estimate how much groundwater the world possesses, where it is distributed and the age of the water in these underground reservoirs. http://news.yahoo.com/map-worlds-groundwater-shows-planets-hidden-reservoirs-140313649.html