From BiblePortal Wikipedia

King James Dictionary [1]

UNDERNE'ATH, adv. under and neath. See Nether.

Beneath below in a lower place.

Or sullen Mole that runneth underneath.

The slate did not lie flat upon it, but left a free passage underneath.

UNDERNE'ATH, prep. Under beneath.

Underneath this stone doth lie. As much beauty as could die.

Webster's Dictionary [2]

(1): ( prep.) Under; beneath; below.

(2): ( adv.) Beneath; below; in a lower place; under; as, a channel underneath the soil.

International Standard Bible Encyclopedia [3]

un - dẽr - nēth ´ ( תּחת , taḥath , "the bottom (as depressed)"): "Underneath are the everlasting arms" (  Deuteronomy 33:27 ). In these words Moses sums up the history of Israel and gives expression to his final thought about life and time and all things visible. Underneath all phenomena and all the chances and changes of life and time there is unchanging law, everlasting principle, an all-enfolding power, an all-embracing love.