what to do with an abondoned well casing, found below grade level.
There is about a 20 foot hole eight inches in diameter.

The best thing is not to tell anyone. Depending on where you live
there may be some pretty stringent regulations relating to properly
abandoning a well. They don't want surface water to be in contact with
ground water (contamination issue). Aside from that you could just
fill it with gravel.

The specifics depend on where the well is located. However, it is a
good idea to seal the well if it is not in use.  Open wells can cause
ground water contamination, especially if the casing is below grade.
You realize the importance of this if your domestic water comes from a
private well nearby. Even if your water does not come from a nearby
well it is possible that somebody is using water from the aquifer.
Some states such as Minnesota provide grants or cost sharing to help
pay for sealing the wells. The link below provides information
regarding unused wells and who to contact for cost sharing assistance.
 Whatever State you are located in is likely to have similar
information available.
http://www.health.state.mn.us/divs/eh/wells/abandwel.html