Grand Prismatic Spring

Grand Prismatic Spring lies in the Midway Geyser Basin of Yellowstone National Park, in north-western Wyoming, beside the Firehole River between Old Faithful and Madison. It is the largest hot spring in the United States and the third largest in the world, after Frying Pan Lake in New Zealand and Boiling Lake in Dominica.

Fur trappers probably saw it in 1839, when one party wrote of a boiling lake in the area, and the Washburn-Langford-Doane Expedition passed by in 1870. Geologists of the Hayden Survey named it in 1871 for its vivid colours, which reminded them of light split by a prism.

The spring measures about 370 feet, or 110 metres, across and is about 160 feet deep. It discharges around 560 US gallons of water a minute at about 70 degrees Celsius. The centre is a deep blue, the natural colour of very clear, deep water. Around it, rings of green, yellow, orange and red are formed by mats of heat-loving bacteria and archaea. Each colour band reflects the temperature where those microbes thrive, and the outer bands tend to look orange and red in summer and darker green in winter. Steam rising from the surface often catches the colours.

A boardwalk loops past the spring and its neighbour Excelsior Geyser Crater at water level, while a short trail climbs to the Grand Prismatic Spring Overlook on the hillside, which gives the classic view from above.

The spring is included in the Yellowstone entrance fee: 35 US dollars for a private vehicle and all passengers for seven days, 30 dollars for a motorcycle, or 20 dollars per person on foot or by bicycle for visitors aged 16 and over. Non-US residents aged 16 and over also pay a 100 dollar nonresident fee each, which is waived for holders of an annual or America the Beautiful pass. Midday on a sunny, windless day gives the best colours, as cool mornings produce thick steam. Stay on the boardwalks at all times, as the ground near hot springs is thin and the water can cause severe burns. Parking at Midway Geyser Basin fills quickly in summer.