“Since nobody did we can only assume that they all perished from some calamity.”
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The “calamity” could have been ordering all of the workers to be killed, and then the killers were, themselves, killed by others who had no idea where the tomb was located. I will grant that it was certainly a calamity for those who were killed, but I doubt that it was a big natural disaster or anything like that.
That’s a reasonable theory... Hire somebody to kill the help... Then kill the help.
If I ever become a pharaoh someday I’ll need to remember that.