This troublemaker baffles even articulate speakers. And they know it. If you listen closely, you'll notice people trying to save face by fudging the last syllable.
Phenomenon is singular. "Management is a universal phenomenon," declares a business website. It helps to remember the -on on the end, which almost spells one.
The plural form is phenomena. A commentator on national television had it exactly backward. He spoke of "the phenomena of climate change" and later used phenomenon as a plural. Others say "phenomenas" when they mean phenomena.