Underneath
Underneath is one of the nine levels of the Underworld.
First named in The Yellow Knight of Oz, ch. 7
The Underworld is divided into nine levels. First there is Neath and Underneath; then Low and Below.
Where this is
placed Beyond the desert worked out
read out of the chapters cited on this page, not stated in any one sentence of them
One level of the Underworld reached by boring straight down from Long Island - a fairy realm beneath our world, not Earth geography itself and not inside Oz.