>>530189843
It's gaster's prison, which sans had to lock him away in because his discovery of (you) completely broke him, driving him insane and making him a societal threat.
Carol and undyne go to the bunker daily to feed him and that's about it.
Sans is in town specifically because somebody had broken into the bunker and communed with gaster, and he's trying to fix the (you) problem before anybody gets hurt.