Ok on the darklands I am goiing off menmry here but, It was made long ago by something called vault builders. And much like earth the deeper you go the hotter it gets. There is also radiations to deal with in spots.
On the drow, well ya see in the age of darkness when the big rock stuck the world elves kinda left, went elsewhere. The drow staied went deep became more corrupt more evil and "turned" ya see drow are not just evil they are twisted evil each city worships a Demon lord and the there are no good drow. In fact and elf can be come so corrupt so vile twisted and pure evil he can change into a drow.
On gnomes gods I love the PF gnomes. They are fey , or used to be anyhow. They do not age as most races they"blech" become pale and age like that. They always are living life to the fullest, learning new things joking having hobby's they obbsess over one thing as if they start to settle start to not have new experiences they die. You louck a gnome away and he dies "of old age" in a year.
Gnomes's can smile and move there face muscles in ways other races can not has the structer is not the same. It can distrube some folks. They can have skin and hair and eye any color found in natue and is not limited to the partents coloring. They had no language when they came from the first world but made up there own by taking words from other languages that they enjoyed the sounds for and assigning it to something. . I mean the Gnome word for tree could be the old azlant word for shoe. The words for Mug, beer and shirt could have been elven words for sword, pants and tree
