Geothermal energy, long a relatively untapped form of power, has suddenly become hot.

Produced by drilling deep underground and extracting steam or hot water through tiny fractures in rock, geothermal power has always been an attractive source of renewable energy because it can be generated round-the-clock, unlike wind and solar power. The problem: Geothermal resources can be difficult to find and expensive to bring into production. As a result, only about 5% of the nation's renewable power supply, or about...