tachath, “under, below,” strong’s H8478

The first occurrence.

And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so. Gen 1:7

The primitive root.

Strong’s H8478  תחת  tachath, a preposition meaning, “under, beneath;” from an unnamed, unused primitive root meaning, “to depress.” The 3-letter root of the preposition is tav + chet + tav.

tav ת = crossed sticks, thus mark, sign, signal, monument
chet ח = the wall, thus outside, divide, half
tav ת = crossed sticks, thus mark, sign, signal, monument

The story: That which is marked (tav) as above is divided (chet) in the middle from that which is marked (tav) as below.

THE ANCIENT HEBREW LEXICON | INDEX OF HEBREW ROOTS