The Gates of The Under

The Gates of The Under is a partly sentient ghost wall with multiple doors. It's only seen at certain times, and only by those who have (almost) died or are from the underworld themselves; however, it is possible for anyone outside of those groups to enter it without knowing, but they will be transported to a random location instead of the underworld - causing multiple worldwide disappearances.

It spans across the world, which is why it looks like it's endless. Its doors are adorned with skeletons with glowing eyes.