Merriam-Webster Collegiate Dictionary - phenomena

 noun  (plural -nas)  Date: 1576 nonstandard phenomenon Usage:  Phenomena has been in occasional use as a singular for more than 400 years and its plural ~s for more than 350. Our evidence shows that it is primarily a speech form used by poets, critics, and professors, among others, but one that sometimes turns up in edited prose the Borgia were, in modern terms, a media ~ — Economist. It is etymologically no more irregular than stamina, agenda, and candelabra, but it has nowhere near the frequency of use that they have, and while they are standard, ~ is still rather borderline.