Let's get dirty. In childhood, the back yard, the flowerbed, the beach, the mucky place where land slips into puddles, lakes, and streams are infinitely fascinating. It is a mistake to leave that "childish" fascination with mud and dirt behind. The soils of the Earth, whether underneath our feet or pressurized beneath tons of ocean water, hold life in abundance. A handful of garden dirt may harbor more species than the entire aboveground Amazon. The robotic rovers Spirit and Opportunity made headlines as they scraped their way across the Martian landscape, searching for signs of life. But while our eyes have been turned toward the skies, teeming beneath us and largely unexplored lies what Science magazine recently called the true "final frontier." A growing array of scientists is exploring life in soils and sediments, uncovering a living world literally alien to our own senses - and yet one whose integrity turns out to be crucial to life above ground.