"The steam... was of three distinct colours: white, red and light sky blue. I am unable to say (why) and shall leave the explanation to some scientific tourist who may... visit this place in the future."
Thus a fur trapper, Osborne Russell, who spent the 1830s and '40s in the western United States, first described this miraculous sight. In 1871 it was named the Grand Prismatic Spring by an expedition led by Ferdinand Hayden, director of the US Geological Survey. People thought the watercolour sketches made by the expedition's artist, Thomas Moran, must be exaggerations, so in 1878 geologist Albert Peale returned to verify them.
What is it: a volcanic eruption, a sunspot? How big is it: a puddle, a sea? What caused it: a natural catastrophe, a nuclear detonation? What's happening to it: is it growing, does it always look like this? In fact, this is the largest hot spring in Yellowstone National Park, which straddles the states of Idaho, Montana and Wyoming. At 90 metres across and 50m deep it is the third largest hot spring in the world (the two largest are in New Zealand). Water flows into it evenly from all sides at 2,000 litres a minute, forming a series of small terraces.
The area around Yellowstone has a violent past and, in all probability, a violent future. Catastrophic volcanic eruptions rocked the region 2 million years ago, 1.2 million years ago and again 600,000 years ago. This last eruption resulted in the park's central portion collapsing to form a caldera (basin) 45 kilometres by 70km. The heat from this eruption was so great that it still powers Yellowstone's geysers and springs. The park contains more than three-quarters of the world's geysers and an extraordinary selection of geothermal features and landscapes.
The Grand Prismatic Spring is in the Midway Geyser Basin, immortalised by Rudyard Kipling in 1889 as "hell's half acre". Its colours are the result of light, heat, bacteria and algae. In the centre of the pool, the water is 87xC, too hot to support life. The brilliant blue colour is the result of refracted sunlight. Away from the centre the water is cooler, although still too hot to touch. In 1967, Thomas Brock, a scientist at Indiana University found that the yellow, orange and red colours are biomass pigments produced by single-celled photosynthetic organisms (cyanobacteria) called thermophiles, which thrive above the known heat limit for photosynthesis. That discovery led to the recognition that there is a third kingdom of life: the archaea, midway between bacteria and the eukarya (higher animals and plants). Why yellows, oranges and reds? The principal pigment of photosynthesis is chlorophyll, which is green, but here it is masked by carotenoids, pigments related to vitamin A, which protect against bright sunlight. Thermophiles are found in hot environments in many places - from volcanoes to power plants - but nowhere are they found in such brilliant colours as here.
The colours change from season to season. In the summer, when the sun is at its brightest, the carotenoid content is high but in the winter the more subdued sunlight results in less carotenoid and more chlorophyll, which changes the colour to shades of green. A spell of cloudy weather in the summer can have the same effect. Brown algal mats form along the cooler edges.
The Grand Prismatic is studied for many reasons. In addition to its geological interest, the hot spring environment is thought to be similar to that on Earth when life evolved. It may even hold the key to understanding where to find life in our Solar System: if single-celled organisms can survive here, perhaps they can survive on Mars or elsewhere. Cyanobacteria form the base of several food chains, which are slowly being understood.
Yellowstone Park contains some of the few undisturbed geyser basins in the world: springs in New Zealand and Iceland have been tapped for geothermal energy. At the moment, the area is under pressure from developers, with restrictions on geothermal exploitation around Yellowstone failing to secure Congressional approval. Visitors also cause pollution, accidentally or carelessly and, as a by-product of using hot springs as wishing wells, coins and other items can block the narrow vents of geysers. Yellowstone was the world's first national park, established by Congress in 1872, but it will need good friends if it is to survive.