Oil

But while old wells might be forgotten, they never go away. After decades, this plugged well started to leak.

"Grass didn't grow," Burns said. "The pine trees kept dying. And there was another tree that sat there, and it died."

Leaks from defunct oil and gas wells are a problem for more than just plant life. The chemicals that leach up from deep underground, including benzene and arsenic, can harm human health and leach into water reservoirs. And any remaining natural gas that works its way into the atmosphere contributes to global warming. Natural gas is almost entirely composed of methane, a greenhouse gas that's far more potent than carbon dioxide.

Millions of oil and gas wells have been drilled in the U.S. since the oil industry was launched in the 1800s. A small number of those might conceivably be repurposed for something else — making geothermal power, for instance. But the vast majority will someday need to be plugged.

Doing that properly is neither easy nor cheap. And if plugging isn't done properly, it might not last — as Burns discovered.