Phenomenon is singular. Phenomena is plural. Although many Greek and Latin plurals eventually give way to English forms, phenomena is one of the few that is preserved by convention, and many English speakers consider it the only correct plural. Of course, there would be nothing wrong with the English plural, phenomenons, but it has yet to gain much ground on the traditional plural.
You might also be interested in the proper plural form of cacti or cactuses.
Both phenomena have garnered a decent amount of coverage over the last year. [Guardian]
I’m talking about the modern phenomenon of the booty call. [Stuff.co.nz]
All the above phenomena are due to the continuous increase in liquidity preference. [The Economist]
Lady Gaga did not become a worldwide phenomenon because she’s normal. [Globe and Mail]