Core Concept: Lava tubes may be havens for ancient alien life and future human explorersSometimes, there’s more to a lava flow than meets the eye. Beneath a fresh, sterile, and steaming hot surface, molten rock can still be chewing its way into the ground, carving caves that can stretch dozens of kilometers. On Earth, such lava tubes (once cooled) are a challenge for spelunkers. On the moon and Mars, these features are piquing the interest of planetary geologists, astrobiologists, and explorers. Lava tubes on the moon and Mars—larger than this one in Iceland’s Surtshellir-Stefanshellir lava tube system—could provide habitat for spacefaring colonists as well as opportunities for geological exploration. Image credit: Under Earth Images/Dave Bunnell. Besides providing a window into geological history, lava tubes offer environmental conditions that are relatively stable and likely to be more hospitable than those found on a planet’s surface. This may make the tubes appealing to life-forms of all sizes, from microbes to spacefaring colonists from Earth. If Mars ever hosted life, it may have moved into such refugia as the planet evolved and surface conditions became increasingly harsh. Indeed, some researchers suggest that microbial life may yet hang on in the Red Planet’s underground havens. “On Mars and other places, lava tubes have the potential to have made the difference between life and death,” says Pascal Lee, a planetary researcher at NASA Ames Research Center in Mountain View, CA. Wherever lava tubes are found, they’ll be scientifically exotic, says Lee. And if a mission to another world is designed to explore such an underground feature as well as the surface, “it’ll be like getting two planets for the price of one,” he notes. Lava tubes can form almost anywhere molten rock streams from the ground. Steady flows of low-viscosity lava are the most likely to form lava tubes. They can form in a way akin to how rivers …