The organisms in the group are very small and can only be seen using a microscope. They are called microorganisms.

Bacteria are microorganisms classified by their shape. They can be rod-shaped, spheres or spiral.

Archaea are a distinct domai of prokaryotic organisms; they have distinct similarities, but also differences, with bacteria.

Archaea have distinct similarities to eukaryotic cells, suggesting that they are more closely related to eukaryotes than to bacteria.

Archaea were originally found in extreme environments but have since been found in seawater, soil, and even in the intestines and dental plaque of animals.

Extremophiles are organisms that live in "extreme environments," under high pressure and temperature. Bacteria often form on the rocks near the hydrothermal vents.