Worm Sign

"Usul, we have wormsign the likes of which even God has never seen!"

—Stilgar, Dune

The Monster is coming for you, but you can't see it. What you do see is the floorboards splitting as it pushes through the ground underneath them towards the camera, or just dust being kicked up if it's outside. It's not always played for scares though; sometimes it's just a convenient visual shorthand for indicating where something moving underground happens to be.

Nothing whatsoever to do with Movie Sign, Wormtongue or Wormtail.

Examples of Worm Sign include:

Anime and Manga

In the fourth episode of the original Bubblegum Crisis OVA, Sylia and Linna travel through subway tunnels at high speed and the road above them splits as they pass.
Occurs in One Piece when Miss Merry Christmas travels underground.
- This also happens when Captain Kuro moves faster than the eye can see–scratch marks appear on the walls where he's running.
3x3 Eyes featured at one point an invisible creature with three clawed legs; when it attacked, all you could see was three rows of claw-marks being rapidly carved to the floor.
In the 23rd episode of Petite Princess Yucie, Diabolos' evil roots attack in this manner.

Comic Books

A series of cryptic illustrations by Chris Van Allsburg includes a picture of a terrified man cornered in a room, rearing a wooden chair up to strike an unseen thing moving toward him beneath the carpet. The title and caption read "Under The Rug - Two weeks passed, and it happened again."

Film

The Dune film
Towards the end of John Carpenter's The Thing, the thing races underground towards McReady, splitting floorboards as it goes.
Tremors
Western horror film The Burrowers. The titular creatures attack from under the earth, and then drag their prey down into it. They do this because, like vampires, they are vulnerable to the sun.
Subverted in Aliens. You don't see the floorboards move till it's too late, you just hear the beeping of the motion detectors.
The American Godzilla remake does this when Godzilla arrives from the ocean. The pier splits most satisfyingly.
In The Frighteners the Big Bad evil ghost does this to various surfaces (wallpapers, rugs) in a haunted house.
Scorponok in Transformers.
- The Driller in Dark of the Moon. Big wormsign.
He Who Walks Behind the Rows in Children of the Corn. He doesn't walk, and He's between the rows, not behind them.
Freddy at the beginning of A Nightmare on Elm Street 3: The Dream Warriors
Wormsign on the moon marks the approach of the Kelium, ancient, burrowing robots with probing tentacles intent on cutting humans to pieces and using them for spare parts in the Walter Koenig/Bruce Campbell vehicle Moontrap
The emergence of the first tripod in the 2005 War of the Worlds is preceded by splitting roads, collapsing buildings and bursting water pipes.
Screamers
At the end of The Incredibles, a giant wormsign appears in the parking lot right before the Underminer's giant Drill Tank pops up and he starts ranting.
The gopher from Caddyshack left ridges of disturbed earth all over the golf course.

Literature

Dune, the Trope Namer
In Stephen King's IT, Pennywise once approaches one of its victims in this way.

Live Action TV

A good Buffy example may be the giant worm which leaves tunnel patterns in "Beneath You".
The Stargate SG-1 episode "The Scourge" has the flesh-eating insect plague do this.
The Silurian scorpions in Primeval.
- In 4.4, Becker tracks a creature in a cafeteria by how it knocks over chairs.
Probably Tremors: The Series. Not sure, to busy being mad about Farscape.
One episode of Sliders had them sliding to a universe inhabited with a beast that did this.
The Dune miniseries.
Happens in a season 2 episode of Fringe.

Tabletop Games

The Rattlers from Deadlands are named so because when they make wormsign, people's teeth rattle from vibration.

Video Games

Mostly averted in Jedi Knight: Jedi Academy; you barely see a puff of dust to denote the movement of the Sand Worms. You can see exactly where they are with Force Sense, though, but you have to be close, because that power is at its lowest level that early in the game.
The tunneling power in Champions Online leaves a Bugs Bunny-esque burrow trail to show your position.
The Thresher Maw in Mass Effect 2 uses this to confuse you as to where it will pop up next.
- And the epic Thresher Maw in Mass Effect 3 demonstrates this while chasing Wrex's convoy.
  - And against a freaking Reaper.
The giant green wormoids from Zombies Ate My Neighbors! do this as an obvious tell for when they will attack, as a direct nod to Tremors.
World of Warcraft has tunneling worms who do precisely this. They're slightly more annoying than most enemies because while you can see them, you can't damage them until they burst out of the ground.
Various enemies in The Legend of Zelda series do this.
In The Suffering, the embodiment of death by live burial does this when it chases after you. However, it leaves behind no permanent trail, and when it digs out of the ground to attack you the hole it creates will vanish.
- However, if you're quick, you can throw a bomb down the hole.
In Sonic and Knuckles, Sandopolis Zone, there are disturbances on the surface of the quicksand, marking the location of the wormlike robots. The disturbances stop right before they leap out.
Super Smash Bros. Brawl does this with Rayquaza, the second boss of its Subspace Emmisary mode. When Rayquaza uses Dig, the ground gets cracked and pushed upward to show where it will attack from.
Time Splitters: Future Perfect; Tremors-like worms at the beginning of the Big Boo's Haunt level.
Castlevania: Curse of Darkness has a class of undead that burrow in this manner before leaping out of the ground to strike at you.
The Dune computer games, including Dune II. The first Dune game ups the ante by including static discharge from the sand, meaning the worms are followed by bolts of lightning.
The moles in Game and Watch game Vermin.
You can do this in Golden Sun 2 with the Sand Psynergy, which turns you into a big pulsating mound of sand, free to move around, but incapable of attacking. Played straight with the Scorpion King boss, who swims around underground leaving a wake and one Earth Djinni.
Armageddon 2, a map-pack for Skulltag, has this done by its twin bosses in the "Sand Worm Trench" level. They pop up occasionally from under the sand to breathe fire at you, and spend the rest of the time under the ground, raising one hell of a dust cloud.
Among others, Blos-type monsters in Monster Hunter do this. Usually, by the time you see the dust clouds being kicked up, it's a bit too late to react - the real purpose of the dust clouds is to let you figure out exactly what the monster's attack pattern is, so you can avoid it from then on.
In Shadow of the Colossus, Dirge, the "Sand tiger" has this when approaching the player character from a distance.
The Mechworm boss in Heavy Weapon. If you see sand being thrown up under you, get out of the way or face a One-Hit Kill.
Metroid Prime 2 Echoes features this with the boss Amorbis.
Both the Driller and Mole enemies from Mini Robot Wars have this when under the ground.

Webcomics

In Goblins, a network of cracks spreads across a wall towards an enemy who's attacking Dies Horribly. As the attacker pauses to notice this, two spiky, burrowing outgrowths from Dies' shape-changing arm burst out of the cracking wall and stab him.

Western Animation

Bugs Bunny does this as his method of travel.
Wallace and Gromit: Curse of the Were-Rabbit
SpongeBob SquarePants takes this trope literally in the episode "Sandy, Spongebob, and the Worm"—Sandy is tracking a giant Alaskan Bull Worm and finds a small wooden sign with "worm" written on it.
Gopher from Winnie the Pooh can often be seen doing this.

Real Life

Some insects and other small critters like moles and sidewinders can hold their breath and "swim" under the surface of soft sand for a short time.
Small crocodiles and large snakes in muddy lakes.
Any homeowner can tell you that moles really do leave trails like this on the surface over their burrows.

This trope can also be seen in Dune.