I have never had that happen. I have reached it with two of the boreholes gone and the third one about to be eaten by a worm though. So probably you are having trouble with worms.
Worm distruction of boreholes seems to be more of a problem if you have them unexplored within your own territory than if they are outside of it.
Of course, if you play again and have the same problem, you may have a brand new bug named in your honour.