Where does “ὑπόγειος” come from?

ὑπόγειος (Ancient Greek) comes from Ancient Greek γῆ, from Ancient Greek γαῖᾰ — the Earth; land, earth.

ὑπόγειος (Ancient Greek): underground, subterranean, chthonian

Definitions

ὑπόγειος traces back through two separate lines of ancestry.

Step	Language	Word	Meaning
1	Ancient Greek	γῆ	land, earth; country; soil
2	Ancient Greek	γαῖᾰ	the Earth; land, earth

Step	Language	Word	Meaning
1	Ancient Greek	ὑπόγαιος	—
2	Ancient Greek	γαῖα	land, country; earth; the Earth