The Great Vowel Shift

"If you think English spelling is confusing — why "head" sounds nothing like "heat," or why "steak" doesn't rhyme with "streak," and "some" doesn't rhyme with "home" — you can blame the Great Vowel Shift. Between roughly 1400 and 1700, the pronunciation of long vowels changed. "Mice" stopped being pronounced "meese." "House" stopped being prounounced like "hoose." Some words, particularly words with "ea," kept their old pronounciation. (And Northern English dialects were less affected, one reason they still have a distinctive accent.) This shift is how Middle English became modern English. No one is sure why this dramatic shift occurred. But it's a lot less dramatic when you consider it took 300 years. Shakespeare was as distant from Chaucer as we are from Thomas Jefferson."

Taken from 25 Maps that explain the English Language
http://www.vox.com/2015/3/3/8053521/25-maps-that-explain-english