During the summer of 1977, a severe drought throughout the western part of the United States caused the north arm of the Great Salt Lake, Utah, to evaporate to its lowest level in a number of years, resulting in the precipitation of about 99.6% pure sodium chloride. At the extreme north end of the north arm, in the vicinity of Monument Point, low humidity combined with a shallow‐bottom gradient to form a salt crust along the lake margin in which gas‐emitting domes were common. The domes varied in size; each contained a large amount of gas that analysis showed to be 82 to 86% oxygen and 14 to 18% nitrogen, with <0.2% or no methane, and no detectable carbon dioxide. The bottom of the crust of the domes (3 to 4 cm thick) was a bright red color. Microscopic examination of this red zone revealed 3.5 × 104cells ofDunaliella salina,2 × 103cells ofD. viridisand 108to 109clumps of red halophilic bacteria per g of salt. Measurements in situ showed sufficient light (34 klx) and temperature (34°C) under the salt to favor the growth of the algae and associated bacteria within the water phase of the crust or in water immediately underneath the crust. Oxygen produced as a result of algal photosynthesis was trapped under the crust until the crust was either eroded by waves or broken under pressure, releasing the gas. The domes were not observed during the summers of 1978 and 1979.