going under (Noun) — (of a ship) sinking.
going under (Verb) — Go under. ex. "The raft went under and its occupants drowned"
going under (Verb) — (astronomy) go down below the surface (esp. of water) to become submerged. ex. "the sun goes under early these days"
going under (Verb) — Be called; go by a certain name. ex. "She goes under her maiden name again"