go under in WordNet English dictionary

1. be called; go by a certain name
   "She goes by her maiden name again"
   go by  go under 
   
2. go under, "The raft sank and its occupants drowned"
   sink  settle  go down  go under 
   
3. disappear beyond the horizon
   "the sun sets early these days"
   set  go down  go under