Pentaphthong

Fifty-six diphthongal combinations may be made from the eight vowel elements, four of which, though, we find to be necessarily triphthongs, two tetraphthongs, and one a pentaphthong.

Try saying the Dutch word Leeuwen some time. At least a pentaphthong...

Someone once claimed that Pres. Coolidge ( a Vermonter) pronounced the vowel in "cow" as a pentaphthong.

Phonetically, [j] is meaningless, it's the same as an [i]. So what we have here is a pentaphthong: ui@iu.