Dwarves have to wear special containment suits in order to protect them from the forces of the surface. Dwarves do not particularly like discussing this and it is a very private matter to them, so it is considered rude to talk about. The legend is that the ancestors of the dwarves made a pact with the fey so that they could burrow deep into the earth and this pact came with the curse of never being able to return to the surface again.
Dwarves refer to the surface world as overburn.
If a dwarf is exposed to the surface for too long they “go derro”. This is a condition where they grow unstable due to being too close to the surface and their physical form will start to mutate wildly (tentacles, eldritch horror, etc) while also going feral. They are highly dangerous when derro and it can be a real problem if they are allowed to exist in this state. There is no cure or way to reverse a dwarf that’s gone derro.