worth a read, page-38

Hi Jaybird,
I'm no GEO, but seeing as nobody has commented on your post I'll have a shot (please someone correct me if i'm wrong!):

These voids are commonplace amongst deposits of this nature (and IMO should be considered a blessing!). They are usually (er.. um.. i believe) created by various materials reacting, forming other compounds, etc in the earth and it is one of the indicators that IVR look at in guessing what lies around the corner.

They pose difficulties with drilling (losing bits, core loss, etc) yet they may be interpreted as a good sign when in the presence of high grade zones.

I doubt they will lose any trucks down such holes and no, they are not the spot where some crafty Spaniard has swiped IVR's rightful spoils under the cover of night.