Primitive single-celled organisms thought by some to be the oldest life-forms on Earth. They were first known for their ability to survive in a range of extreme environments, such as salty water, hot springs, and sulfur vents at the bottom of the ocean floor, though further research also finds Archaea in a variety of non-extreme environments. There is some disagreement as to how the Archaea should be classified. They were originally categorized as a sub-group of Bacteria, but they have very few similarities with them. Some taxonomies place them in a domain of their own, based on the differences between their genetic code and those of the Eukaryotes and Prokaryotes. Others place them within the Monera. Thus, the main divisions within the Archaea (Crenarchaea, Euryarchaeota, and sometimes others such as Korarchaeota and Nanoarchaeota) are variously described as kingdoms, phyla, or even classes.