There are places where the Earth's inner rumblings burble to the surface — like Iron Spring, located just outside the bustling town of Manitou Springs in central Colorado.

Every few seconds, a burst of water surges out of a narrow pipe, splashing into a concrete basin that's partly dyed a bright orange.

"Whenever I see that color, I look very carefully because sometimes it's not chemistry that's forming that rust. It's biology — an entire world of unexplored and undiscovered microbes", says James Henriksen , an environmental microbiologist at Colorado State University.

Henriksen uses a metal file to sample the microorganisms teeming invisibly and improbably in the colorful puddles atop the basin. "Just like birders are constantly looking for birds," he says, "I'm constantly looking ar

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