Across international and US campuses, a new sustainability trend is taking root: “swap shops” and free stores, designed to reduce dorm-room waste while easing students’ financial burdens.

In the struggle against climate change, it’s easy to wish for some technology—or even magic!—that could remove Earth-warming carbon dioxide (CO2) from the atmosphere and turn it into something useful. But this mechanism already exists and is much older than the climate changes of the past two centuries.

On a chilly morning in Zurich, commuters rush through the humming arteries of the city’s underground train system. Unbeknownst to them, the air shimmering behind the tiled tunnel walls holds a secret: Beneath their feet and beyond the blaring trains, the Earth is offering up energy.