Google searches for this term do not turn up much. A “mortal hole” is most often used to describe a hole in rock made for mortar and pestle use by Native Americans. It is also the name of a rock band, a 4-second YouTube video (which is accompanied by a short story, ending with this line (typo theirs, meaning unclear to me): As our lips met, I knew that the empty whole inside my heart was meant to be mortal.),
Also the search turned up the phrase as a line from a Jean Genet story (Funeral Rites) denoting a bullet hole, and makes a cameo appearance in lyrics for a song called ‘Shelter for my Soul’ by Powderfinger:
When I’m released from this mortal hole
I’ll take my leave but I don’t wanna go
When fate delivers me all I’m askin for,
A place to rest and shelter for my soul.