Once thought to be rare “extremophiles” confined to boiling hot springs and salt lakes, Archaebacteria (or Archaea) are now recognized as one of the most abundant life forms on Earth. Far from being niche survivors, they are a dominant force in the planet’s biosphere, numbering in the nonillions ($10^{30}$). Their population is so vast that, while they contribute less biomass than bacteria, their carbon footprint is roughly equivalent to that of all animals on Earth combined.

The distribution of Archaea is heavily skewed toward environments hidden from human eyes. The majority of their population resides in the deep biosphere—the terrestrial and marine subsurface buried kilometers beneath the ocean floor and soil. Here, in lightless, energy-poor conditions, they form massive, slow-growing communities that may have persisted for millions of years.

In the open ocean, Archaea are surprisingly prolific. In the deep pelagic zones (the dark waters below 200 meters), they often outnumber bacteria. A single group, the Thaumarchaeota, is so abundant that it acts as a primary engine for the global nitrogen cycle. Even in the human body, Archaea are present; the species Methanobrevibacter smithii is a common resident of the human colon, where it aids digestion by consuming waste hydrogen.