How to determine the severity or size of a hole in yard?

I found a hole in my yard. This area has three common features that could cause that, a sink hole from an underground stream, prairie dogs, which are everywhere in the neighborhood, or an abandoned mine shaft. For safety reasons, I need to determine how large the hole is. It is 4 ft from the property line, so I also want to know how much of my property is endangered, but want to be careful examining the hole, as it could be more on the neighbors side (it does point that direction), and so disturbing it could undermine their property.

What can I do to determine the size of the hole?