Sometimes you want to do a bit of armchair exploring to get inspired. This is things that I have read or seen and recommend.
UE Magazine (2012-)
NY Times article "The Wilderness Below Your Feet"
featuring Erling Kagge, Steve Duncan and Andrew Wonder
Crack The Surface and Crack The Surface II
Urban Explorers: Quests for Myth, Mystery and Meaning
Urban Explorers: Into the Darkness (IMDb)