There is a long-standing view in ecology that species richness is maximal in the tropics and gradually declines towards the poles. However, it appears that this is only true for aboveground diversity. Belowground (in the soil), clear relationships between latitude and species richness do not seem to be so clear. Furthermore, there are 27% areas of mismatch between aboveground and soil biodiversity. Indeed, a single soil profile may contain equivalent diversity to that found aboveground within an entire ecosystem.
Wow factor: The mushroom-forming ‘honey’ fungus Armillaria ostoyae is the largest living organism on the planet by area, estimated by scientists as a contiguous specimen found in the Oregon, USA covering 965 hectares (2,385 acres), equivalent to 1,350 soccer fields. The mycelium of Armillaria ostoyae grows and spreads primarily in the soil, out of sight. This fungus causes Armillaria root disease in a number of trees, particularly Douglas-fir, grand fir and subalpine fir. Wow!
Estimated global distribution of soil biodiversity. Orgiazzi et al (2016) Publications Office of the European Union (https://www.sciencedirect.com/science/article/pii/B9780128096659098220)