Look at this video I captured during our recent trip to Yellowstone NP, WY. This Old Faithful is the most predictable geographical feature on earth. Water erupts almost every 91 minutes. Last year we missed it. When reached this place, it had just erupted and we didn't have time to wait another one and half hour. This year we were lucky. We just had to wait ~15min. Details of this phenomenon can be found in wikipedia.