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Beautiful job! Your idea is reminiscent of the book "All the Light You Cannot See" by Anthony Doerr, which I recommend highly!
This is exceptionally cool! One minor thing: Depending on how well you know your lawn, you should consider calling your utilities provider[s] before digging to ensure you don't strike a buried gas, power, water, sewer, or comm line. Most municipalities have a "call before you dig" service that will mark all underground utilities for free. Just a safety thing. Carry on.