That $500MOne reason the initial underground capital cost is so large is that it does 10 years of mining work up front, before the first shovel load of ore is even processed.
Whereas in open pit you gradually descend, paying the stripping and blasing costs as you go, in the long hole shrink method the whole kimbelite pipe is 'scrambled' in situ first.
They start at the bottom and cave upwards until the whole of south lobe has been blasted, effectively creating a silo of kimberlite rubble in the host rock.Thereafter it's continuously scooped out at the bottom and trucked to surface.
The bottom-most ore is naturally drawn off first and because it is relatively richer, the pay-back period is relatively shorter than it otherwise would have been.
https://www.lucaradiamond.com/site/assets/files/5485/ni-43-101-karowe-ug-fs-technical_report-final-2019-12-17.pdf