The region between Terra Cimmeria and Terra Sirenum features several enigmatic knob fields located in large degraded basins. These knob fields were first mapped on Viking imagery as chaotic terrain. However, they lack a direct link to an outflow channel and also show other differences to the chaotic terrains of the circum-Chryse region. Therefore, the terms knob field or knobby terrain are used despite their official names like for example Atlantis chaos further to the east, or Caralis Chaos presented here. These regions are believed to represent the remnants of the ancient Eridania lake. Spanning over roughly 1.1 million square kilometers and three large basins, lake Eridania may represent the source region of the large Ma’adim Vallis outflow channel further to the north. The Eridania lake is hypothesized to have existed during the Middle to Late Noachian period. Later in time, the large lake divided into smaller isolated lakes and eventually disappeared altogether along with the rest of the water on the planet.