I'm a big fan of Max Action and his Minneapolis/ St. Paul based urban exploration site Action Squad. They are a semi-organized group of young folk who explore abandoned and otherwise hidden areas of the urban environment. The area is riddled with underground tunnels due it's unique geology, particularly St. Paul, and Max and his crew have explored most of what you can get to, and some of what you can't.
The writing on the site is what distinguishes it from other urban exploration sites: "We'd explored our way there, through the Labyrinth, without the benefit of maps, rumors, or any idea of what, if anything, we might find around the next corner. We'd hauled over a hundred pounds of gear deep into the earth, into the guts of a building that has not existed for decades, in order to drop down into an old manmade cave system that had lain buried, undisturbed, and almost totally forgotten for god knows how many years."
Max continues to explore, and is now getting kind of existential. Check out the exploration of his own crawlspace, and what he found there. Read about his trip to Seattle, and his finding of a rubber duck on the seashore.
The best part of the site is the section on The Labyrinth. There's a wealth of interesting exploration narrative on the site, and there are still occasional updates, but the Action Squad site is something to find and read, then find again and re-read much later. Which is what I've done, and why I'm sharing it with you.
Explore Whidbey Island.