The Atlantis II Deep is a 65 km2 topographic depression located in the axial trough of the Red Sea at 2,000 m depth. The depression traps 17 km3 of hot and dense brines fed by hydrothermal fluids. This chapter reviews numerous data collected during the last 50 years. Chemical and isotopic data suggest that the processes that lead to the formation of the Atlantis II Deep brines are similar to those that produce open ridge black smoker fluids, but the recharging fluid is sea water in the case of sediment-free ridges, whereas it is sea water that has dissolved evaporites in the case of the Atlantis II Deep. The monitoring of temperature indicates that the heat flux was 0.54 × 109 W between 1965 and 1995. After 1995, the heat flux became 10 times lower. The substratum of the Atlantis II Deep consists of MORB-type basalts, which are covered with 0- to 30-m-thick metalliferous sediments. The solid fraction contains biogenic calcareous and/or siliceous components and silico-clastic detrital particles diluted by metalliferous sediment, which consists of metal oxides, sulphides, carbonates, sulphates, and silicates that precipitated from the hydrothermal fluids. The redox interface between the deepest brine layer and sea water is a major place of mineral precipitation. During glacial periods before the Holocene, the redox boundary was located above the brine–sea water boundary, so that hydrothermal metals spread over a large area of the Red Sea bottom.< Leer menos