preposition / adverb
beneath
bih-NEETH/bɪˈniːθ/
In or to a lower position than something.
The red ball rests beneath the wooden chair's seat.

Etymology
The prefix made the idea of 'below' refer more clearly to where something is, rather than where it comes from—hence a position lower than something else.
In context
beneath the fish's head was a human head, and beneath the fish's tail were human feet