USA 5: Yellowstone 3: Geyser Basins

Old Faithful is definitely the most popular geyser in Yellostone, if not in the world. It erupts quite reliable about every 90 minutes. It is located close to the parking lot and all important facilities such as restrooms, restaurants and gift shops. This results in a lot of people waiting to see its eruption as you can see on the next picture. I won't put up any pictures of the geyser itself as it is rather unspectacular compared to the others in the park. A cool thing is that for all the predictable geysirs there are estimates shown in the visitor center.The Castle Geysir is only about 15 minutes walking distance from Old Faithful and there is only a fraction of the people there.

Another cool thing are the hot pools. In there live different microorganisms that are specialized on certain temperature regions. So the colour of the pool can be interpreted as a thermometer, only the scale is different than we are used to. The cooler (or rather less hot) regions are red and the hotter, almost boiling regions are blue.
Grand Geyser is the tallest predictible geyser on earth. The eruptions reach about 60meter (200ft) which is an impressive fountain.