I like Atom's comment about concealed mine shaft's.
Even though you can see open shaft's, You do not know
what is 6 inch's under your feet.
Yes, The ground can cave in under you and some of those holes or stopes if
you wish can be in the 100's of feet deep.
One way trip mate. And it is not the fall that kill's you,
It is the sudden stop at the other end.
Best to stay away from a known mine shaft area until you gain a lot
more experience. Even folk's who are highly knowledgeable still get
caught at times.
There is a video on here some where showing a fellow happily detecting away
and when he starts to scratch for a target, A small hole opens up.
That area is now fenced off from both the cattle and people.
.
The famous Poseidon Lead near Laanecoori is now caving in every where.
It is so dangerous that my Uncle warns people away from the area.
Have a look through these photo's.
It will give you an idea what to expect.
http://home.vicnet.net.au/~tarnagul/photogallery/gallery3/index.php/mining/DSC_1787