Nearly 5 million people visit the Grand Canyon each year, and millions more descend on Yellowstone and Yosemite, the remaining stanchions of America's national park triumvirate. But the majority of these visitors, as many as 90 percent by some counts, rarely stray much farther than their cars or the conveniently placed scenic overlooks. Another small percentage signs on with commercial outfitters offering exploratory trips within the parks. But the truth is, no one will take you deeper, literally and figuratively, into Yosemite, Yellowstone, and the Grand Canyon than those parks' nonprofit field institutes - the Yosemite Association, the Yellowstone Association Institute, and the Grand Canyon Field Institute (GCFI). Nearly as much of a secret as the overhang upon which we stumbled, these field institutes, once specializing in brief, mellow ranger-led walks, have morphed into an outdoor lover's dream, upping their adventure quotient to test the mettle of even the most experienced backpacker, while offering that backcountry lover something he wouldn't get on his own, a traipsing educational experience like no other, courtesy of wilderness experts with PhDs in biology, geology, and the like.