A makhtesh (Hebrew: מַכְתֵּשׁ ([maχˈteʃ]), plural: מַכְתְּשִׁים ([ˈmaχtəˌʃim]) – Makhteshim) is a geological landform considered unique to the Negev desert of Israel and the Sinai peninsula of Egypt. 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 diverse fauna and flora. The best known and largest makhtesh is Makhtesh Ramon.
Although commonly referred to as "craters," these formations are "erosion cirques" (steephead valleys or box canyons). Craters are formed by the impact of a meteor or volcanic eruption, whereas makhteshim are created by erosion.
Where a hard outer layer of rock covers softer rocks, erosion removes the softer minerals relatively quickly, 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. 
The center of the Negev is dominated by northeast-southwest anticlinal ridges. The crests of four ridges host five deep valleys surrounded by steep walls. The upper half consists of hard limestone and dolomite and the bottom of friable sandstone. Each valley, known as a makhtesh, is drained by a narrow river bed.
The Negev has five makhteshim: Makhtesh Ramon; Makhtesh Gadol; Makhtesh Katan; and two small makhteshim on Mount Arif, south of Makhtesh Ramon.
- Makhtesh Ramon is exceptional as it is drained by two rivers (Nahal Ramon and Nahal Ardon). It is the largest makhtesh at over 40km long, 2–10km wide and over 500m deep. The rocks in this makhtesh contain thousands of ammonite fossils, as well as volcanic and metamorphic rocks.
- Makhtesh Gadol (Large Makhtesh) At the time of naming, Makhtesh Ramon was uncharted and so this was thought to be the largest makhtesh, at 5km by 10km.
- Makhtesh Katan (Small Makhtesh) is the smallest major makhtesh at 5km by 7km and was charted in 1942 by Jewish explorers.
The two makhteshim in Sinai, Egypt, have no names for the basin, but their walls have several names including Jabal al-Manzur or Gebel Maghara.
- Mazor, Emanuel and Krasnov, Boris, editors "The Makhteshim Country - a Laboratory of Nature". Pensoft Publishers, Sofia, 2001, 411 pages. ISBN 954-642-135-9
|Wikimedia Commons has media related to Makhtesh.|