Mouse Hole

The cartoon mouse's refuge: a neat arch-shaped hole cut into a wall at the floor, sometimes with a door.

If you have one of these in your house, expect your perfectly triangular cheese wedges to go missing soon.

Often the gate to Mouse World.

Film

 * A rare live-action example, though more realistic looking, can be found in the movie Mouse Hunt.
 * It also appears in Willard, created by the giant rat Ben chewing through the wall.

Literature

 * Again, not mice, but The Littles use this trope.

Live-Action TV

 * Episode 20 of Monty Python's Flying Circus has a ratcatcher looking for rat holes in a house. The first one he finds is a Mouse Hole, complete with fake mice on a string.
 * There is a mouse hole in Mr. Bean's apartment. In the Christmas Episode, he puts a mousetrap in front with a present on it.

Newspaper Comics

 * A trademark schtick of Garfield strips. Sometimes this even extends to windows with pots of flowers under them, mailboxes, welcome mats, etc.
 * Ziggy

Video Games

 * While they're not mice, the Minish in The Legend of Zelda Minish Cap have several small holes that look like this. They do, indeed, lead to the Minish's Mouse World.
 * In Luigis Mansion, these are all over the place. Some release golden mice, worth major dollars. Luigi can enter most or all with a special device, usually yielding more treasure. Or ghosts. Often both.
 * One of the first puzzles of Sam and Max Hit The Road requires you to grab a stash of cash from one of these.

Western Animation

 * Tom and Jerry, as in the picture above.
 * Early Mickey Mouse cartoons didn't show his house, but early comic strips and children's books did. It had a round, obviously mousehole-like front entrance despite Mickey's large, non-mouselike size.
 * In 1931, Mickey's house began to be shown in the cartoons and had a normal front door there, after which the ancillary material was retconned to match.
 * The "Pixie & Dixie and Mr. Jinks" shorts on The Huckleberry Hound Show
 * Likewise the Looney Tunes cartoons featuring Speedy Gonzales and, before that, Babbit and Ratstello.
 * Sniffles lived in a mouse hole in some shorts as well, even earlier.
 * Mostly averted in Walt Disney's Cinderella, but there is one.
 * My Little Pony Friendship Is Magic: Fluttershy may have purposefully installed a mouse hole in her house to shelter mice.
 * Invoked in Tex Avery's King-Size Canary, when the jumbo-sized cat and mouse do this with a train tunnel (@ 7:02).
 * You might notice the classic arch-shaped mouse holes in the floor boards in the background on The Simpsons.