Reddit user atomicbolt cited the song "Summer Nights," which says that when Danny and Sandy met on the beach, he "saved her life, she nearly drowned." Sandy sings that "he showed off, splashing around," but there's no word on whether or not Danny successfully saved Sandy from drowning.
According to atomicbolt, "Sandy did actually die on the beach that day."
"As she drowned, her brain deprived of oxygen, she had a vivid coma fantasy involving her summer fling Danny, where they shared a magical year of high school together," the theory states.
Essentially, while drowning, atomicbolt says, instead of her life flashing before her eyes, she dreams of what would've been the perfect future.
"The visions get increasingly outlandish as time passes, until finally, as Danny desperately tries to resuscitate her on the beach, she sees herself flying into Heaven in her dying moments."
The mark of her earthly departure, the theory says, is when Danny and Sandy ascend happily ever after up to Heaven in a gravity-defying convertible.
"The entire movie was a drowning woman's coma fantasy," atomicbolt concludes.
"Everything that happens in the last scene is just a little TOO perfect," another Reddit user wrote. "Danny and Sandy are back together despite everything, Rizzo SUDDENLY isn't pregnant AND Kenickie suddenly decides that he actually loves her, the geeky kid gets onto the sports team, EVERYTHING is suddenly okay, just the way that sweet, innocent Sandy would have wanted it to be."
Theorists also point out that the last line of "Look at Me, I'm Sandra Dee (Reprise)" is "Goodbye to Sandra Dee."
Another theory goes on to say Danny died too.
"Danny tried to save a beautiful stranger from drowning but lost his own life as well in the process," the commenter wrote, suggesting Danny invented the name "Sandy" because they met on a beach. "He envisions what his life would have been like if he had rescued her."
"Grease" director Randal Kleiser hasn't commented on the theories.