"under the rock" is correct and usable in written English.
It is often used to describe something hidden or sheltered away from view. For example: "She found a rare beetle living under the rock."
I'm afraid to look under the rock".
Three months into his tenure, Todd peeked under the rock.
We don't want to look under the rock that was Jackson's real life.
Continue until you pass under the rock arch, where you turn right.
Viewers will be able to walk under the rock and peer up 15 feet at its underside.
He doesn't have many other options unless he can get out from under the rock of this scandal".
If he has finally found a true home with the resurgent, out-from-under-the-rock Los Angeles Clippers, as the best shooter on the N.B.A.'s highest-scoring bench, well, it has taken all of his 32 years to get there.
Two years beforehand — I am reliably informed that it was two years, although I spent most of the movie under the rock-hard impression that it was a matter of weeks, if not days — he loved a dancer named Lisa Diane Krugerr), who vanished without so much as a beep on his voice mail.
Under the rocks are the words, and some of the words are theirs.
At low tide, kids can also busy themselves searching for crabs under the rocks.
When there is direct fire from there [across the border], one cannot hide under the rocks.
