get under (something)
Literally, to move or position oneself underneath or below something. The kids got under the covers, and I started reading their bedtime story. Ugh, the mouse got under the bookshelf somehow—we'll never catch him now!
get under something
to get beneath something. The cat came in and got under the sofa where she couldn't be seen. Why don't you get under the table where Billy won't find you?