Benchmark Hunting is a spin off of a sort of geocaching. There are many survey markers used by the NGS which were placed way back when and can still be found today. They are actively used in surveying so when you find one don't take it just snap a pic or record your find. Most of them are disks or monuments and are found all over the place. There are many near my hometown and some of them have not been found for years.

Learn more about Benchmark Hunting here: http://www.geocaching.com/mark/

See the benchmarks I've looked for
See the benchmarks I've hid