Door to Nowhere

This prompt is a seed for the planting round of SeedComp 2025, to be (hopefully!) used in the sprouting round. 

Photo taken in Wissahickon Valley Park in 2018. The (apocryphal) cave of Johannes Kelpius is further south and better known, but this mysterious door whose purpose I've long since forgotten is probably more in the spirit of what we're doing here.

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