There are two ways to get to Mossbrae Falls, one of the most beautiful (and most often Instagrammed) natural attractions in California, about 30 minutes from the base of Mount Shasta. Unfortunately, neither of the routes are legal — and the more popular one involves a potentially deadly trek along active railroad tracks. And yet the ethereal beauty of the falls is still a powerful temptation for visitors, who come here by the thousands, often drawn by shots splashed across Instagram and other social media sites. Tall as a five-story building and longer than an Olympic swimming pool, Mossbrae is...