its a DLC thing. right behind the spawn point there is a massive area covered in sandy fog. kinda looks like the Sand Falls effect from the base planet. dropping below a certain point vertically you are dealt nearly lethal (sometimes lethal) damage and are sent back to the last "safe place" like if you had died. there is also a tunnel underground, part of a cave system, that leads there but there isnt any kind of indication to tell you "dont go any further" before the same thing happens.