In this serene corner of Java, farmworkers pick tea leaves from striped-green hills threaded with pipes. The three-foot-wide pipes carry steam from a broiling underground reservoir, a reminder of the volcano that once erupted.

Here, along the volcano-rich Ring of Fire, the Wayang Windu Geothermal Power Station uses the Earth’s heat to produce energy that emits few or no greenhouse gases. It aims to expand, because it’s discovered what may be the world’s biggest hot steam well.

“We have a very big resource,” says plant engineer Boyke Bratakusuma, as he points to the increasingly deep exploratory wells that Jakarta-based Star Energy is drilling in the shadow of the Wayang and Windu Mountains.

Indeed, this archipelago nation has the world’s largest known geothermal reserves, and it desperately needs clean energy. Now the fourth most populous country, it’s developing so rapidly that a quarter of its people still live in homes without electricity. Awash in cheap coal, it relies on coal-fired power plants for nearly half its electricity and plans to expand that usage.