"Phenomenons" and "phenomena" are both appropriate plural forms of this noun. "Phenomena" should be used in scientific or philosophical contexts. "Phenomenons" is only appropriate outside of those contexts.
It is a growing social phenomenon on high school campuses.
The phenomenon was predicted by Einstein.
The phenomenon was first noticed by...
[T]here are, among other natural curiosities, two unusual formations of land. Twenty miles from the city a pair of enormous eggs, identical in contour and separated only by a courtesy bay.... They are not perfect ovals..., they are both crushed flat at the contact end.... [A] more arresting phenomenon is their dissimilarity in every particular except shape and size. I lived at West Egg, the — well, the less fashionable of the two, though this is a most superficial tag to express the bizarre and not a little sinister contrast between them.
F. Scott Fitzgerald -- The Great Gatsby
The concept of unity, in which positive and negative are attributes of the same force, in which good and evil are relative, ever-changing, and always joined to the same phenomenon—such a concept is still reserved to the physical sciences and to the few who have grasped the history of ideas.
Arthur Miller -- The Crucible
Some readers will find the story unreal and disturbing, but child abuse is a disturbing phenomenon that is a reality in our society.
Dave Pelzer -- A Child Called It
The phenomenon has most reliably been reported in times of crisis or in stress situations,
Stephen King -- Carrie
The dryness and dullness through which your patient is now going are not, as you fondly suppose, your workmanship; they are merely a natural phenomenon which will do us no good unless you make a good use of it.
C.S. Lewis -- The Screwtape Letters
All down the ages there have been countless reports of strange phenomena-poltergeists, telepathy, precognition-which you had named but never explained.
Arthur C. Clarke -- Childhood’s End
...the luminous state of the atmosphere ... was a phenomenon upon the duration of which we could calculate with certainty.
Jules Verne -- A Journey to the Center of the Earth