The Enigma of Amigara Fault

A short horror manga by Junji Ito, located in the back of the second volume of his fish-zombie-apocalypse book Gyo. A boy (Owaki) and a girl (Yoshida) meet on Amigara Mountain, where an unsettling discovery has been made. Human shaped holes are scattered across the mountain, and it soon becomes clear that the holes are "calling" to the people they are shaped like. So what happens when they enter the hole? Well, you can be sure that massive amounts of Claustrophobia and Nightmare Fuel are involved. Can be read here.

Should it seriously disturb you, counter with liberal application of this comic, or this one, or this one, or this one., or possibly even this one, this one, or this one. There's also a dramatic reading.

Tropes used in The Enigma of Amigara Fault include:

And I Must Scream: It's implied that no matter how long you spend inside your hole, you can't die. Some of the noodle people that come through might wish they were dead.
Body Horror
Catapult Nightmare: Provides the trope page image.
Claustrophobia: You will have it by the end of the story.
Daylight Horror: Most of the creepy stuff happens at daytime.
Downer Ending: Despite Owaki's attempts to calm down Yoshida and give her a reason to not enter her hole, complete with using heavy stones to fill the hole, she does anyway. Owaki is too big to follow; he succumbs to the compulsion after seeing his own hole. Only then does the government close off the site, and try to find the cracks on the other side. What they do find is not pretty.
Dreaming of Things to Come: Or dreaming of the past and present. Owaki dreams that he is Nagatsuka, a man who entered the mountain the day before. He also has what feels like a memory, as a condemned prisoner being sentenced to death by walking into a mountain. These dreams disturb Owaki when he awakens.
Dying Like Animals
Fate Worse Than Death: Let's just say that if you get the compulsion to enter your hole, you may wish that you would succumb to starvation and dehydration. Whatever comes out of the other side is still alive, and in great pain.
Hope Spot: When Yoshida finds her hole, she is scared, to say the least. Owaki tells her to calm down, and finds heavy stones. He proceeds to fill her hole with the rocks, pointing out that with no way to get inside, it can't make her travel that far. Then he stays with her that night, asking about her family and bonding with her. The compulsion gets her the next day, so that by the time Owaki awakens, she has already removed the stones and entered her hole.
Noodle People: Literally - when people enter "their" hole, they are stretched in every direction until their limbs are thin like tentacles, yet are still alive.
Oh Crap: The ending has the mountain scientists doing this remark when they see what's coming thorugh the cracks.
Primal Fear: This manages to hit isolation, darkness, the unknown, claustrophobia, and body horror all in one fell swoop.
Schmuck Bait: A prime in-universe example. Owaki and Yoshida came to see the holes because they saw them on television,a nd Yoshida swears that she found her own hole.
Shown Their Work: As per the dream, there were people in Japan as early as 50,000 BC. They also developed stone-carving technology remarkably early.
Token Romance: It was really unnecessary for Owaki and Yoshida to get together. It happened so that we'd get a giant Hope Spot, before things get worse.

"It's slowly coming this way!!" DRR... DRR... DRR...