Being an occasional series in which an expert archeologist answers vexatious questions sent in by ignorant members of the general public.
As an Archeolgist I am often asked: What is a post hole and is finding one bad for your back? This is not an easy question to answer - or at least the second part of it isn't. To take the simpler matter first: a post hole is a hole in the ground made by a post. Being, on the whole, of undateable but almost certainly ancient origin, generally speaking the hole is empty since the post responsible for making it will have been of wooden construction and is likely to have rotted away leaving little or no trace - especially in the boggy conditions in which most post holes are found and most posts are not.
A more difficult and more nuanced question - and therefore one more likely to be put at an international gathering of distinguished archeologists such as my self, rather than by an ignorant inquirer such as my current virtual interlocutor - is: is a post hole always a post hole in that it has, without any shadow of a doubt, resulted from the insertion into the ground of a post, or could it just be a hole - deliberately made, for example, or quite possibly even inadvertantly created as it were - by an object other than a post for a purpose or purposes other than those expected of a fully functioning well post-holed post?] To put the matter more succinctly: when is a post hole not a post hole? A well-considered answer might take the following form: when it is a hole made by something else. We call such holes, quasi-post holes, faux post holes, non post holes or, more radically, holes.*
* I deal with this matter at greater length in my 2018 article "Post Holes, Non Post Holes and Non Post Modernism - A Stochastic Approach to Post Hole Distribution Patterns in Prehistoric Europe", Dorset Post Holes Quarterly Vol XXXIV. The interested reader will be surprised by the variety of holes identified via this complex mathematical approach: currently 212 and counting.
Turning to the second and more difficult part of the original question, my answer must, perforce, be relativistic. Though there is relatively little danger of falling into a post hole [they tend to be smaller in diameter than the average modern foot] and though the absence of an actual post means that injuries resulting from the extraction of an ancient but wedged post are extremely rare, bending down to inspect a post hole [often with a view to determinig their "post hole" v "hole" status, can result in unforseen spinal trauma, especially if the observer is of a certain age and build and suffers from compromised flexibility. Repetative strain is also a constant danger if numerous holes are involved in a single site inspection, whilst deeper holes have a higher than average discovery:hospitalisation ratio than their shallower cousins.
For the general public - as opposed to the highly skilled archeologist - this generally suggests that a cautious "leave it to the experts" stance is advisable especially when approaching an unidentified post hole; whilst the presence of dogs, small children and unexploded devices, usually denotes that an advisory recommendation should be upgraded to a "do not disturb" notice. Metal detectors are reminded that few post holes contain metal-detectable remains [ven with the machine set to "wood"] and the possession of metal detecting equipment is no guarantee of safety.
Is an empty sherbert fountain wrapper a reliable indicator of settlement continuity?