Great Wall of America (5e Hazard)
From D&D Wiki
This wall can be any size, shape and appear anywhere, The Wall Is immune to all magic damage and all piercing, bludgeoning and slashing damage from any material or weapon and will become 1d10 feet higher each time it is attacked. It is unaffected by any spell, attack or effect. It's purpose is to keep many different races out of a specific region, and so the Wall is generally extremely long.