The Iberian Pyrite Belt in southwestern Spain looks looks like a movie set for an alien world. Rusty lakes punctuate the iron-rich landscape. The Rio Tinto, named for its vibrant red coloration, seems to glow against the dull rocks. But dig a little deeper, and things get even weirder.

In a surprise to scientists, cyanobacteria have been found thriving nearly 2,000 feet below the strange landscape, where sunlight, water, and nutrients are scarce. Researchers previously thought these microbes could survive only while basking in the sun's rays, although they are otherwise a versatile bunch; researchers have found them alive nearly everywhere on Earth.

“You go to the desert, you have cyanobacteria; you go to the sea, you find cyanobacteria. You go