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bottomless rule
(lemm.ee)
Be sure to follow the rule before you head out.
Rule: You must post before you leave.
there is the Kola Superdeep Borehole, I think a Soviet Union experiment to find out how far they could drill, over 12 KM down (which is not quite through the earth's crust).
Not bottomless, but far enough to get that sense of bottomlessness. Not very wide, though.