The legendary Kola superdeep borehole is about 12 km deep. To achieve such depth, a major difficulty is to make sure the borehole is vertical straight.
Intuitively, when the borehole is deep enough, it is very hard to make sure that the drilling bit is still moving downwards. Suppose it hits some hard rock, it can be easily deflected.
So, how straight is the Kola borehole? What technique is invoked in drilling straight downwards?