For nearly four decades, Wang Guiling's office has been the most volatile terrain on the planet, from the boiling geysers of Yunnan to the high-altitude hot springs of the Xizang autonomous region and western Sichuan province.

Wang, a researcher at the Institute of Hydrogeology and Environmental Geology in Shijiazhuang, Hebei province, affiliated with the Chinese Academy of Geological Sciences, is fascinated with the origins of thermal heat.

"It's not just a landscape," said the 62-year-old, referring his geological surveys. "It's a conversation the planet is having with itself."

At the core of Wang's work is one of humanity's most pressing questions: Where will our energy come from when fossil fuels run out?

Geothermal energy is almost entirely carbon-free, and carries none of the meltdown risks associated with nuclear power and none of the intermittency problems faced by wind or solar energy.

"Solar energy is an inexhaustible cosmic energy source, while geothermal heat is Earth's eternal gift from within," he said. "As long as the sun shines and the planet keeps moving beneath our feet, these two will never run out."

Wang's passion for his work goes far beyond earning a salary, in fact, in the 1990s his institute struggled to do exactly that.

"I never wavered. I genuinely love this work. It comes from inside," he said.