This has always been the most common depth, and if you were on the USGS site it says somewhere that estimating depth is hard, it's often off by a kilometer, and when they are in doubt they default to 10 km (which of course helps solidify 10 km as the most common depth. Where did this idea come from that there are tunnels this deep anyway? Why would anyone construct anything that deep, down in an unstable zone? How did these tunnel borers get deep enough in the first place to set off nuclear weapons without people noticing? When you think of all the crime that is done and gotten away with in ordinary basements, it seems pretty odd to try to go deeper than any known manmade shaft.