Paul Bellis Jones left a one-use camera near the summit of Mount Tryfan. Along with the camera he left a note, which instructed hikers to take a picture and then leave it for the next hiker. Jones did this as an experiment to prove to his friend that people can be trusted. After four days, the camera filled with photos of over 30 hikers was returned to Jones by a National Park warden.

Even if you can't trust everyone, you can at least trust hikers.

Link to article and photos.