Phenomenon is a singular noun ^{1 2}. Phenomena is always plural ¹. Phenomena has been in occasional use as a singular since the early 18th century, but singular phenomena is primarily a speech form used by poets, critics, and professors, among others, but one that sometimes turns up in edited prose ³. Merriam-Webster’s Dictionary of English Usage suggests that the singular phenomena is no more irregular than the words "agenda" and "stamina," which are clearly accepted as singular ⁴.