This is exactly what I've always wanted to do. Belinda Bain bought a mountain in Big Bear Lake, CA and built a zipline so she could glide down the side of it at 45 mph. The project reportedly cost Bain a cool million. No wonder she's charging $95 for a three-hour tour.
Video Bonus: No, it's not Bain's zipline. But this South African zip looks pretty incredible.
h/t: The Goat