Link: https://www.nationalgeographic.com/environment/article/arbuscular-mycorrhizal-fungi-networks-map

Scientists created the first-ever map of this vast underground fungal network and found it could stretch to the sun and back more than a billion times.

Global topsoils hold an estimated 110 quadrillion kilometers of fungal network, a distance equivalent to traveling to the sun and back a billion times.

The study: Global density and biomass of arbuscular mycorrhizal fungal networks

Thanks for providing the study link.
Fortunately I work at a university so can access it.

Why are the geographical boundaries of Poland so easily recognizable on the map of funghi density?

Might have something to do with this: https://scienceinpoland.pl/en/news/news%2C109008%2Cpoland-still-among-europes-most-polluted-particulate-matter-rises-expert-warns

Hmm perhaps, although I still wouldn't expect the border to be so well visible on the map. Because diffusion doesn't know political borders