Hypogeal

Hypogeal means "underground".

In botany, a seed is described as hypogeal when the cotyledons of the germinating seed remain non-photosynthetic, inside the seed shell, and below ground. The converse, where the cotyledons expand, throw off the seed shell and become photosynthetic above the ground, is epigeal.
In water purification works, the hypogeal (or Schmutzdecke) layer is a biological film just below the surface of slow sand filters. It contains microorganisms that remove bacteria and trap contaminant particles.