Yellowstone Park is one of the most places in the USA. There are thousands of hot and . It is visited by millions of every year. Only recently have they begun to understand what they are looking at. Yellowstone Park is the largest active in the world. It covers the of almost 4,000 square kilometres. The last big caused the from the volcano to cover most parts of the North America and the change of the . During the winter, clouds of dark ash hid the sun. It was so cold that many of animals and plants disappeared from the Earth. The volcano is still alive today. Tests have shown that thousands of happen every year. They remain since they are very weak. However recently the number of earthquakes has and in some parts the of the ground has risen. In the last 2.1 million years there have been three eruptions. Most experts believe there is no danger of another giant , but it’s impossible to be sure.