Mendenhall Glacier is a spectacular 12-mile-long glacier near Juneau, Alaska. Ice caves make up its surreal interior. Only the truly adventurous can access its icy walls, as the trip to see them requires a kayak trip, an ice climb and—once you're inside—the faith that the glacier’s melting walls won't give way. But this natural wonder is under threat. The glacier has been retreating incredibly quickly in recent years due to warmer temperatures and rising sea levels. Since 1958, it has receded by almost two miles. What remains, however, is an utterly breathtaking fantasyland.