Seems like a nice fable. The Real Problem is there is zero evidence of any hole or ditch or quarry of any kind (even the side-shaved kind) massive enough to acocunt for the Smithsonian castle...
Seneca Mill and Quarry Ruins
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Deep in the heart of Poolesville, MD, lies the captivating Seneca Mill and Quarry Ruins, a place steeped in history and mystery. The remnants of this once-thriving industrial complex offer a glimpse into a bygone era, where the echoes of bustling activity still resonate through the crumbling walls and weathered structures. Visitors are transported back in time as they wander through the overgrown paths and explore the abandoned buildings, imagining the workers who toiled here decades ago.
Surrounded by the tranquil beauty of nature, the Seneca Mill and Quarry Ruins stand as a silent tribute to the industrious spirit of the past. From the imposing stone walls of the quarry to the rusted machinery of the mill, every corner tells a story of labor and ingenuity. As the sun filters through the trees and illuminates the worn facades, one can't help but feel a sense of awe at the resilience of these structures against the relentless march of time. A visit to this historical landmark is not just a journey into the past, but a moment of reflection on the enduring legacy of those who came before us.
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Reviews

Ruins of a quarry from the late 1700s and stonecutting mill from early 1800s. To see the ruins go upstream from Seneca Creek Aqueduct a hundred yards until you come to a trail on your right....