Virtual cache
From GeoWiki
A Virtual Cache is a Geocache that is placed where it wouldn't be appropriate or possible to place a physical Geocache.
A Virtual Cache usually requires photographic proof and an email to the cache owner. Most often, the owner will want some piece of information supplied that will prove to them that you visited the site.
Piggy Strikes Gold is an example of a Virtual Cache.
They were originally designed to allow a cache to be placed in an area where a physical cache is not permitted or appropriate. Some cache listing websites allow them while others such as Groundspeak are tightening up on their requirements.
