New NASA satellite data revealed Earth’s largest underground aquifers, which are a source of fresh water, are being depleted at alarming rates, according to research published Tuesday in the Water Resources Research Journal. Research shows 21 of the 37 largest aquifers located around the world have passed a sustainability tipping point, which means more water was taken out than replaced during the 10-year study period. The new research underscores major concerns about the reliance on underground aquifers, which supply 35 percent of the water used by humans worldwide. Drought-stricken California is using aquifers for 60 percent of its water use, compared to typical use of 40 percent. The study points out the emptying aquifers are exacerbated by human activity such as agriculture, growing populations, and mining.