Originally Posted by
ORAC
As the article states the depth is comparable to those drilled by the USA for their underground tests - and remember the shaft has to be wide enough to accommodate a a nuclear warhead and its associated test equipment.
It's not as simple as drilling a shaft and backfilling on top of the device, either; if you do that, it turns out that the explosion just shoots all your backfill back up the shaft. You need to excavate sideways from the bottom of the shaft too; there's quite a good writeup of the last US test, with a drawing of the shot cavity here:
https://en.wikipedia.org/wiki/Cannikin