Living under a rock is a nice recent English idiom meaning “being oblivious or ignorant to what happens in the outside world”. It is used to describe a person who doesn’t know something any “normal” human being is supposed to know, usually in relation to some recent events:
This guy lives under a rock—he doesn’t even know that George Bush isn’t the president of the United States anymore.
It is most commonly used in a question (not very polite), in amazement that someone doesn’t know something the speaker considers obvious:
Have you been living under a rock? How can’t you know that Peter and Jane got married?