Sentence:  "Whew!" said Tom, his eyes opening wide. "What must I do when I find a <WSD>sand boil</WSD>?" / "If you see a boil developing, grab a sack of dirt—we'll leave some at points a few hundred feet apart—put it right over the spout; pile on more as fast as you can and blow your horn for help, and keep blowing. We will get there and attend to it." 
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.