(Photo: Wikimedia Commons via CC Attribution/Share Alike)
Slieve League Cliffs, Donegal, Ireland
If you wish to summit the Slieve League Cliffs, you'll need to take One Man's Path—so named because it is only wide enough for one person to walk at a time. Slip off the narrow walkway and you'll tumble almost 2,000 feet into the ocean below. It's definitely not advised to try this one in misty weather (basically every day in Ireland) or on windy days.