Grand Prismatic Spring

Seen from above, the Grand Prismatic Spring looks like an eye painted by a colourist: a deep blue pupil, a ring of pale turquoise, then bands of yellow, orange and rust-red spreading out over the grey sinter in rays of runoff. It is the largest hot spring in Yellowstone and the third largest in the world, about 110 metres across and 50 metres deep, releasing some 2,100 litres of water at around 70 °C every minute into the Firehole River basin.

The colours are alive. The blue in the centre is simply the intrinsic colour of deep, clear water, strongest where the spring is hottest and sterile. Towards the edges, where the water cools, mats of heat-loving bacteria and archaea take over, and each temperature band favours organisms with a different balance of chlorophyll and carotenoid pigments — greener where it is hotter, orange and red where it is cooler. The mats shift with the seasons, turning richly orange and red in summer and darker green in winter.

The spring was known long before it had a name. In 1839 a party of fur trappers crossing the Midway Geyser Basin described a boiling lake about 90 metres wide; the Washburn expedition passed in 1870, and geologists of the Hayden Survey in 1871 named it for its prism-like range of colours. It shares the small Midway basin with Excelsior Geyser, a steaming crater that pours thousands of litres a minute into the river.

There are two ways to see it. The boardwalk across Midway Geyser Basin takes you to the rim, where the steam often veils the surface and you see the colours at your feet. For the whole spectacle, walk the short trail from the Fairy Falls trailhead, about a mile south of the Midway parking area, which climbs gently to an overlook above the spring — best on a cool, clear morning after the steam has thinned.

Getting there

The spring is in Yellowstone National Park, Wyoming; the nearest sizeable city is Bozeman, Montana, about 130 km away. The nearest airport is Yellowstone Airport (WYS), about 29 km away. Park at the Midway Geyser Basin lot for the boardwalk, or at the Fairy Falls trailhead for the overlook trail (about 1.9 km each way).