As soon as the open pit mine reaches a certain depth, the excavating and removal of the diamond bearing rock from the pipe becomes uneconomical and dangerous.
At this point, the open pit mine is converted to an underground mine but only when the kimberlite pipe continues to produce a sufficient quantity of gem quality diamonds.
Underground mining is done by creating tunnels and shafts in and around the pipe. They are lined with concrete and placed directly under the ore section. In the top of the concrete lining openings are made at regular intervals.
Holes are drilled into the kimberlite and filled with explosives to loosen the kimberlite which eventually falls into the openings.
The broken ore is scraped out of the tunnel by heavy machinery, so-called “scraper drifts” and then loaded into cars on a train that dumps the ore into a crusher which is also underground.
Finally the crushed ore is taken by a conveyor system that carries the ore to buckets. The ore is taken up in the vertical shaft. Now the diamond bearing rock can be further processed.
This mining method is called “block caving “.