Grand Canyon, AZ
This is probably the hardest trail on the list for the simple fact that you are descending into a huge hole in the ground, and then have to climb back out. There’s also little to no water available on the trail, and summer temps reach well into the 100s every day. The hike to the bottom is steep and fast, but offers amazing views of the canyon walls that throngs of tourists rarely get to see up close. Once you complete the first part of the route, rest at the Phantom Ranch, where food and drinks are available. If you make it back up to the top, pat yourself on the back—the Park Service doesn’t recommend that anyone tries to do a rim-to-floor hike in one day due to the dangers of dehydration (and the more than 250 people they had to rescue last year).