Sometimes you want to do a bit of armchair exploring to get inspired. This is things that I have read or seen and recommend.

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UE Magazine (2012-)

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NY Times article "The Wilderness Below Your Feet"
featuring Erling Kagge, Steve Duncan and Andrew Wonder

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Undercity

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Crack The Surface and Crack The Surface II

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Urban Explorers: Quests for Myth, Mystery and Meaning

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Urban Explorers: Into the Darkness (IMDb)