First, the meaning is obscure.
Second, the pronunciation is unguessable.
Third, it's largely Greek, so it probably doesn't mean anything useful.
Fourth, the look of the thing reminds us of hypodermic syringes and congealed things.
(Oh dear...I find myself beginning to feel quite sorry for the poor word...)
Hypogeal (you say it in the worst possible way, HYpoGEEul) means occurring or living below the surface of the earth. I cannot deny that this is quite a respectable sort of a meaning, but that doesn't stop it sounding gluey and patronising. Hypogeal also describes a plant whose first leaves stay permanently under the soil.
There we are. Hypogeal. A serviceable word for describing diamonds, badgers, and peanuts.
Unless you have any taste, obviously.
Word Not To Use Today: hypogeal. This word comes from two Greek words, hupo, which means under, and gē, which means earth.