you are able to build under the map
#build under the map
serene cave
all i needed to get under the map was a stack of 10x10x10 toilet rolls (a big stack of plates works too)
how i started to build there: when i fell down i was able to place bricks, and i only placed the bottom of the wall as i fell, the next time i fell down i landed on the bricks and cancelled the rest of the wall
you can build out of the black box with stairs. its always stairs that are able to be built outside of the map
humble cosmos
Well yes, you can build "under" the map, so you can build basements.
serene cave
Well yes, you can build "under" the map, so you can build basements.
that is smart
i have no idea how the tp back to the ground works. but i have a theory
im guessing the tp back works with "how far have you fallen" so its not always a specific point. i have a video showing it.
at fall 1 i am at the bottom of my stairs and fall down a bit under the stairs.
at fall 2 i jump down at the kinda start of my stair spiral and dont even reach the bottom
humble cosmos
Hmm yes, that warping is a bit sus indeed. 
It looks like the killbox is not working and mking you fall off the world