Board Thread:General Discussion/@comment-26006155-20181123023529/@comment-26006155-20181210032209

Also, you do realize we're talking about time and dimensional travel?

Anyone can go through the barrier, but nobody can leave without both a monster soul, and a human soul. (a hybrid of sorts. Very interesting.. especially if the 7 mages who cast the spell were hybrids)

Just because Chara was the first human who fell through the skylight, doesn't mean she's the first human to visit the Underground.

Remember the time machine behind Sans's house? And how he and Papyrus just.. appeared in the village one day?

I think Sans and Gaster detected a distrubance in the timestream, used a machine to investigate.. and then found they couldn't leave the way they came.