Located at the foot of a mountain just outside of Afton, Wyoming, Intermittent Spring (otherwise known as the Periodic Spring), is one of only three springs of its kind in the world that start and stop every few minutes for a reason science can only speculate on.
A type of natural phenomenon known as a rhythmic spring, there are only three such features known in the entire world, and the Afton spring if thought to be the largest (another being the famed Gihon Spring in Jerusalem). The Afton spring was discovered randomly by a logger who happened upon the strange stream in the course of his work, and has been a natural attraction ever since. The water at the spring site is said to flow for 18 minutes straight, before stopping for another 18 minutes. There may be some variation in these times.
Researchers have not pinned down exactly what causes the surprisingly regular ebb and flow of the spring, but they believe it has to do with a siphon effect that occurs underground. The theory is that a regular spring underground fills up a pipe-like cavity in the stone, creating a siphoning effect when it breaks the surface that rushes the rest of the water out of the "pipe," leaving it empty, and stopping the stream, until it refills and the process repeats. Oxygen found in the water has given credence to this theory.
The Intermittent Spring is most active in late summer, so if you are looking for an interval during which to visit the spring, that would the one.