A bizarre community of microbes has been discovered inside rocks in Yellowstone National Park, thriving in pores filled with water so acidic it can dissolve steel nails. The clusters, interwoven with flourishing green algae, comprise at least 40 different new species of bacteria, according to Jeffrey Walker, a University of Colorado microbiologist — and he and his colleagues say the microbes’ fossil forms could provide powerful clues to the nature of early life on Earth and life that may have existed billions of years ago on Mars.