On the nature of The Underdark on Planet Ocean:
Previous to The Cataclysm, the Underdark was a massive complex of caves and tunnels that spread secretly under much of the world. When the great disaster happened, much of the devestation to the surface was caused when it collapsed into the Underdark, creating the massive surface-spanning ocean of the present.
The only large expanse that survived were the heart of the Drow empire centered under the Southern continent, saved from the floodwaters by the collapsing ice and stone at the edges of the continent, cutting the inhabitants off from the surface for nearly 2000 years. It is only in the last half-century that any communication or travel has been possible between the depths and the surface.
There are ruined caverns and caves and tunnels, remains of a once-great underground empire buried beneath the shifting sands in the North, but few of the inhabitants survived the disaster, and most of them are now great buried necropolises, avoided by all but the foolhardy and the mad when they are unburied. If any parts of the Northern Underdark are still inhabited, none have met them and returned to tell the tale.
There are other portions of the Underdark that lie intact beneath islands or even under the deep ocean, but they are small and cut off, and all manner of strange and dangerous things make their homes within.