Microbes are the smallest known living organisms on Earth and can be found just about everywhere, even in the cold, Mars-like conditions of lava caves. On the island of Hawai'i, scientists recently found a marvelous assortment of novel microbes thriving in geothermal caves, lava tubes, and volcanic vents. These underground structures were formed 65 and 800 years ago and receive little to no sunlight. They can also harbor toxic minerals and gases. Yet microbial mats are a common feature of Hawai'ian lava caves. Samples of these mats, taken between 2006 and 2009 and then again between 2017 and 2019, reveal...