AP Photo

Rudyard Kipling called it “Hell’s Half Acre,” a geothermal wonderland where people could fall through the Earth’s thin crust or be poached by steamy hot springs and geysers. Most visitors to Yellowstone National Park’s Midway Geyser Basin stroll the wooden boardwalks, but a few hike a short, steep side trail that reveals a bird’s-eye view of the entire valley, including Grand Prismatic Spring, which can be fully appreciated only from above. Mustard-yellow and vibrant-orange mats spread like tentacles from the turquoise pool. “Not even the most talented artist could imagine something as beautiful as that,” muses Sandra Banack, a biologist who studies cyanobacteria, the microbes that create the colorful mats — and that hold...