What is the definition of Groundwater Weapon?

A groundwater weapon? Well, imagine if someone used an old-fashioned water gun to shoot down a tank of water from deep underground. That's basically what a groundwater weapon is - a metaphorical way to describe any underground water source that could be used as a surprise attack or weapon, like a hidden reservoir of liquid mighthem.

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Here's an unsettling example sentence: "The villain secretly stockpiled a groundwater weapon beneath the city, ready to unleash an underground barrage of doomsday mud."