Sage Geosystems, a three-year-old geothermal energy startup, is using an abandoned gas exploration well in Texas, as a test bed. Last year, the company created a 3,200-foot vertical reservoir deep underground using its novel fracturing technology. For the last six weeks, Sage has been pumping and storing large volumes of water in the artificial reservoir. Cindy Taff, Sage’s CEO, likened the setup to an earthen battery. The idea is that, during times when wind farms or solar arrays produce more electricity than people need, the excess power can be used to pump water underground. When electricity demand is high, Sage can open up its floodgates and return power to the grid. Sage is one of dozens of companies that are striving to make it easier and cheaper to access the vast amounts of heat beneath our feet – both for producing clean electricity in power plants and potentially for storing energy to keep the grid running smoothly. The results of Sage’s testing to date are very positive, spurring them to move to a full commercial demonstration of the technology.