[@Vesuvius00]I imagine most aquatic monsters couldn't have made it to Mount Ebott on their own, and that the humans might have taken measures to seal them elsewhere. I like the idea of the reservoir, as it seems like a more weighty method to sealing in monsters and preventing humans from ending up trapped with them...though it make it harder for a human to just stumble into that world, it does give a little more overall depth to the other possibilities in general.

Under a major city - higher concentration of boss monsters - the worst criminals get thrown into the underground...feels like the premise of an anime, for some reason
A lost island - same as undertale, except they'd have some surface to live on, create their own civilization
Underwater - large or wholey aquatic monsters
Under a reservoir - more enigmatic monsters - humans sneaking in for ___ reasons