Could the expression unless someone lives under a rock be used in a formal setting? If no, what are some alternatives that could be used instead?
This answer is a kind of extended comment on Kosmonaut's answer. I would have said that "unless you live under a rock" tends to be used with situations where the person who lives under the rock is out of touch with current events. Googling for the phrase gives things like
etc. It's not used so much with things like "circles are round".
I'm not sure how to answer the question you posed since I don't know exactly what "in a formal setting" means.
You want alternatives?
etc. I could easily go on all night.
You would probably not use this in a formal setting, but that is probably due to the fact that it would be considered rude. The problem with suggesting a formal alternative is that the sentiment itself is rude.
Some formal alternatives might therefore be things like: