In 1871, Thomas Moran joined a government survey led by Ferdinand Hayden to explore the Yellowstone region, a place so strange that earlier reports of boiling springs and painted canyons had been dismissed as tall tales. Moran sketched; the photographer William Henry Jackson photographed.

Their images helped convince Congress that the stories were true and that the land deserved protecting. In March 1872, Yellowstone became the world's first national park. That same year Moran finished this enormous painting, and Congress bought it — for ten thousand dollars, a staggering sum at the time.

The colours look exaggerated, but they're not as wild as you'd think: the canyon walls really are streaked yellow, orange, pink and white, stained by hydrothermal alteration of the volcanic rock.

It is a painting that changed a law. Not many landscapes can say that.