Here's something that's been in my mind for a bit....... If the barrier around the underground is a 1 way thing, what would happen if I stood at its edge and just stuck my arm in? Would the arm get stuck? Is it possible to pull it back out? If so what if I just grabbed something or someone from the other side without physically stepping inside the barrier?? WHAT WOULD HAPPEN????
It’s almost time for me to sleep WHICH IS A GREAT TIME TO ANSWER THIS.
My best guess is nothing- there’s no definite line of where the barrier is, when you go to see Asgore at the end it looks like a weird portal that reminds me of the infinite staircase in Mario 64. Monsters might not be able to stand close enough to the exit to get grabbed by someone on the outside. That would answer why they have never been discovered until Frisk fell down.
Anyway, I suspect it depends on whether your soul goes in or not. Obviously human souls naturally have determination higher than monsters.. This is going to get me onto the topic of “Why are resets a thing in the underground and does the concept spread to the rest of the world once the barrier breaks or are they naturally a thing in the real world?” BUT THAT’S FOR A RAMBLE and probably not when I’m ready to fall asleep lol.
To summarize: Nothing would happen and you’d be stuck in the underground and HAVE to make friends with EVERYONE