As the spring melts the winter snow, the full extent of Yellowstone is gradually revealed. Now, from the surrounding lowlands herds of elk, pronghorn and bison return from their winter feeding grounds to takeadvantage of America's richest natural grasslands - right in the heart of Yellowstone. In only a few weeks, a brutally harsh deep freeze has been transformed into a flower-decked nursery perfect for the year's newborn animals. There is also a new cast of characters that emerge bleary-eyed from hibernation as grizzly bears begin to teach their young the secrets of survival in Yellowstone - how to hunt fish in the still-frozen rivers and, as the season progresses, when to move out through valleys and grasslands into summer forests and up into Yellowstone's alpine peaks. In this spectacular wilderness, over 10,000ft high, they slide and scrabble, hunting millions of tiny moths buried under rocks on the barren slopes.

But summer here is fickle; even on Midsummer's Day, winter can descend from the surrounding mountains bringing punishing snows to fragile flower meadows. August is the only month in the year when it does not snow, but then, just as it seems the easy living of summer has finally arrived, it is brought to an abrupt end as fires sweep through the forest, laying Yellowstone to waste.

Yellowstone is the most geothermally active place on Earth. There are 10,000 boiling springs, bubbling mudpots and fumeroles there, and more geysers than in all the rest of the world put together. But for some, nature's fountains are more than just a marvel, they are an obsession. 'Geyser Gazers' have seen them all and can even imitate them. With bated breath, like trainspotters they tut when one is a few seconds late and when another erupts in a new and innovative way they dance for joy.