A waterproof tarp or cloth placed on the ground under a tent to keep moisture and dirt from seeping through.
From ground + sheet (Old English 'scēte,' from Proto-Germanic). Modern camping term that emerged in the 19th-20th century as recreational camping became popular.
The groundsheet is so simple it's easy to forget it's brilliant—by creating an air gap, it prevents capillary action from pulling moisture up through the soil, the same physics that lets tree roots drink water from dry ground.
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