In vanilla, lava fills caves under Y10 - obviously you wouldn't want this in all OTG worlds, but it would be good to have an option for it in the worldconfig.

We currently should have this, but we do need an option to disable it - or potentially to specify a fill liquid, which defaults to lava.

For 0.0.15, added new options CarverLavaBlock and CarverLavaBlockHeight, only applies to OTG carvers.