Under a microscope, tiny tunnels can be seen in mineral particles from conifer forest soil. Scientists believe it is mycorrhizal fungi penetrating these particles by excreting organic acids in order to mine nutrients for their plant hosts. An estimated 150 meters of pores are bored by fungi per year per liter of E-horizon (layer that has been leached of mineral and/or organic content, leaving silicate) soil.
Photo credit: Jongmans 1997. “Scanning electron micrograph, showing 4–6-mm-thick hyphae entering a calcium feldspar at a granite surface near Lunsen, Sweden” [Jongman 1997].
Jongmans, A.G. et al., 1997, Rock-eating fungi, Nature 389: 682-683, https://www.nature.com/articles/39493