Sentence:  The more quiet ejections are apparently more common. These "<WSD>sand boils</WSD>" are produced where liquefied material, mostly sand and water, quietly boils out from its vent, sometimes for two or three days following the earthquake. After the earthquake, a mound of sand remains, surrounding its vent. It is also quite likely that many, perhaps most, extrusive sand features start as sand blows and finish as <WSD>sand boils</WSD>. ... "<WSD>Sand boil</WSD>" also has a local meaning. When the Mississippi, Ohio, St. Francis, and other rivers of the region rise to high stages on their levees, the protected land inside the levee is sometimes lower than the water level in the stream. Because water seeks its own level, this situation produces pressure, a hydraulic head from the higher stream to the lower level on the other side of the levee, causing ground water to move toward the ground surface and produce boiling cones of water and sand. The suggestion has been made, for good reasons, to restrict the term "<WSD>sand boil</WSD>" to these hydrologically-induced liquefaction features (HIL). 
Meaning of the word marked with <WSD> tags:  An eruption of water through a bed of sand due, for instance, to seismic activity, or to differences in water pressure on two sides of a levee which causes water to penetrate beneath the levee.