Lava Lake
Lava Lake is a lake of fire fish in Subterranea where lawbreakers are thrown.
First named in The Yellow Knight of Oz, ch. 7
we usually throw lawbreakers to the fire fish in Lava Lake, and I suppose sky breakers might be called lawbreakers, too.
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
In Subterranea, an underground kingdom explicitly 'reached by the Skyrocket's crash' through the earth's crust, not through Oz or the Nome Kingdom.
What the books say it is
appearance boils 1 chapter
"The deep, dark underwood where the gropers grope, the dragons drag, and Lava Lake boils on forever!"
The Yellow Knight of Oz, ch. 8