-is in Latin and Greek
Several common words in academic English ending in -is come from Latin and Greek, such as analysis, hypothesis and crisis. The 'correct' plural form for these in formal English is -es: in these examples, analyses, hypotheses and crises. Oddly, this form is not correct in the original language. All three of these words (and many others that end in -is) come from Greek, where the plural form would be -eis (kriseis, hupotheseis and so on) - note how strange a strict transliteration is. English adopted these words from their Latin versions, and the regular Latin plural for words ending in -is is -es.
So now it is correct for us to use this incorrect form.