The leader of Underground Railroad Harriet Tubman helped many slaves escape.

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• Karez is a water conservancy system in the arid region of （）, consisting of （） connected by underground .
• The labor dispute turned out that the coal miners decided to ___the pay rise for the sake of greater work safety underground.A.s
• The world’s largest and deepest underground high-speed railway station has beenbuilt 102 meters beneath the Great Wall at the （）