A makhtesh (מכתש, plural: מכתשים - "makhteshim") is a geological landform regarded to be unique to the Negev desert of Israel and the Sinai Peninsula. Although commonly known as "craters" (a literal reading of the Hebrew, and due to the visual similarity), these formations are more accurately described as erosion cirques. A makhtesh has steep walls of resistant rock surrounding a deep closed valley which is usually drained by a single wadi. The valleys have limited vegetation and soil, containing a variety of different colored rocks and a diverse fauna and flora which has been protected and preserved over millions of years. The best known (and largest) makhtesh is Makhtesh Ramon of Israel's Negev desert.
A hard outer layer of rock covered softer rocks. Erosion relatively quickly removes the softer minerals, and they are washed away from under the harder rock. The harder rocks eventually collapse under their own weight and a crater-like valley structure is formed. In Negev and Sinai makhteshim, the hard rocks are limestone and dolomites, while the inner softer rocks are chalk or sandstone. In most cases, makhteshim only have one drainage system, although the Makhtesh Ramon has three.
The Hebrew term makhtesh ordinarily means "crater", and was used to describe these features before the geological formation processes were understood. In fact, proper craters are formed by the impact of a meteor or volcanic eruption. Adopting the Hebrew words allows English speakers to distinguish the two kinds of features; an alternative English term for makhtesh can be "erosion cirque."