On Monday afternoon, two of my great-nephews were about to have the time of their young lives hiking the Hetch Hetchy, a hidden gem in Yosemite National Park’s peaceful northwestern corner. No Starbucks here, dudes. No tour buses or campers and RVs. This is what’s known as off the beaten path. Often called Yosemite’s “secret” valley, Hetch Hetchy’s glacial valley has been inhabited for more than 6,000 years, with its towering granite cliffs and cascading waterfalls. Pioneer conservationist John Muir called it a “remarkably exact counterpart” to the now world-famous Yosemite Valley, about 15 miles south. To Muir, this carved, U-shaped glacier was one of the earth’s most beautiful places.…