I'm fascinated by the biology of soil and the history of "dirtiness"--where dirt and bacteria are allowed to be and where we must clean them away. Mary Douglas defines dirt in her classic book Purity and Danger: An Analysis of Concepts of Pollution and Taboo as "matter out of place":
[Dirt] is a relative idea. Shoes are not dirty in themselves, but it is dirty to place them on the dining-table; food is not dirty in itself, but it is dirty to leave cooking utensils in the bedroom, or food bespattered on clothing; similarly, bathroom equipment in the drawing room; clothing lying on chairs; out-door things in-doors; upstairs things downstairs; under-clothing appearing where over-clothing should be, and so on.
This is part of why the recent news of an expensive French restaurant in Tokyo serving a dirt-based menu is so surprising and wonderful. Fancy food and soil certainly don't belong together, but this is no ordinary dirt; the food-grade dirt is specially sourced and lab-tested to ensure cleanliness, heated, strained, and smoothed into chocolate-like sauces, spooned onto elaborate dishes to give an exquisite "earthiness" to the flavor. These symbols of fanciness and good taste, along with the expensive price tag give the dirt a new place at the table. For more about the dirty menu and the history of geophagy (eating dirt) check out this article from Smithsonian.