'underground'; from Ancient Greek ὑπό (hupó) 'under' and γῆ (gê) 'earth')[1] are biological terms describing an organism's activity below the soil surface.
[2] The converse, where the cotyledons expand, throw off the seed shell and become photosynthetic above the ground, is epigeal germination.
In water purification works, the hypogeal (or Schmutzdecke) layer is a biological film just below the surface of slow sand filters.
The terms hypogean and hypogeic are used for fossorial (burrowing) and troglobitic (or stygobitic) cave-living organisms.
The term hypogeous is used for fungi with underground fruiting bodies - for example, truffles.