What have done towards your prep?

[Mortblanc]

SS I don't think you are going to be able to hit and maintain 40f unless you are digging into a permafrost layer.

Most caves and underground water sources in temperate regions don't vary much from 50f-56f.

Safe temp in most refrigerators is 38f so you are shooting for 2 degrees f above the present refrigeration technology.

Being from the southern US, where there was no way they would cool efficiently, my experiences with "root cellers" has been in the opposite direction, we used them to keep things from freezing during the winter months.

Any natural cooling we got was from "spring houses". Small buildings made from stone that were situated on top of springs were they surfaced. Some foods were kept in containers in the cool water and other foods were kept on shelves on the walls. Temps never went much below 50f, but it felt cool in our 90-100f+ daily summer temps.