Why Fusion is the Key to the Future of Geothermal Energy. Geothermal energy has the potential to power the world. However, it hasn’t been the hottest renewable energy option because drilling deep enough into the earth is difficult and costly. A startup has recently unearthed a solution: A gyrotron heat ray to melt rocks (sort of). This is a well-established nuclear fusion technology being reapplied to geothermal energy. Could it unlock the true potential of geothermal energy and make it a better renewable energy source than solar and wind?