The location of a large group of maars in a basin of 200 km2 in Huitengxile, Inner Mongolia Autonomous Region of China, was probably the center of a magmatic plume in the past. The cross section of maars looks like saucers, with diameters ranging from the largest 900 to the smallest 10 m. Crashed rocks in strata exposed by the drill core and magnetic survey indicate explosions underground by insupportableness of steam accumulation from interaction of magma and water. In Huitengxile and other parts of Inner Mongolia, maars with the same shape and caprock age characteristics exist in an area of 30,000 km2, which indicates that they formed by substantial magmatic activity at the same time and in the same phreatic style. The aquifer layer in the crust constrained the eruption of the magma to overbear via phreatic shooting and releasing its energy. What has been happening at Yellowstone has similar phenomena to what had in Huitengxile and surroundings. The magmatic activities under Yellowstone may have been reined by an aquifer so that magma could not threaten the world.