The most terrifying movie you've ever seen.
 

Okay, so right off the bat, from the title, I'm going to get a lot of well known blockbuster answers. This is not what I want.

I am looking for something underground, something I won't find on a top ten list of horror movies.

I want a movie that will shake me to my core and give me nightmares (I'm an adult, mind you). Not just gore, but really, truly f-ing terrifying.

Please can you help me?

I think that's a pretty subjective question... I have one qualifier, what scares you? I have a unnatural fear of the dark so certain movies get me when people go into dark rooms and you know something's getting ready to go down. I'm not easily frightened but the dark gets me every time. What is your fear trigger?

*The Exorcist
*The Haunting (1963)
*Don't Look Back
Paranormal Activity 1
*The Innocents
Carnival of Souls
*The Legend of Hell House
The Ring
Vertigo
Persona and a few other Bergman movies

The ones with *s are so disturbing to me I would hesitate to watch them again.

P.S. Scary is WAY different than gross/disgusting for me. I've seen lots of movies of the latter sort, but disgust is not an emotion I enjoy indulging. Scary, otoh, now that's big fun ... :-)

Movies haven't scared me since I was a kid watching Pet Cemetary at 3am with metal cooling/expanding sounds coming from the basement. Not sure I can help.

BUT you could try Inhumanities. Its like Faces of Death, but about other things. Live cannibalism, brutality of hunting zebras for their hide. Stuff like that. Its a documentary, not a film with a plot. But not exactly a documentary. Its been a while, but I believe its just sequence after sequence of these things.

You could also try Bug(2006). Watch it with all the lights out, make sure its quiet and allow yourself to be sucked in.

examples
 

Well here's a few examples of movies I consider actually scary:

1. The Fourth Kind
2. Grave Encounters
3. August Underground
4. Oculus
5. Dead End

But I'm looking for something that's not so well known. Psychologically, physically, and or spiritually terrifying.

Im watching a movie now called The House of Last Things that's quite a mind fuck. Its not horror but its scary in a way. It makes your brain twist to follow the story. Also anything by David Lynch, Mullholland Drive, Twin Peaks, Blue Velvet. They are scary but because you get lost while watching them.

Have you seen Yo Momma?

I haven't been here that long but I've seen that question come by several times.
"Hey, what's the scariest/most shocking/most disgusting/whatever horror movie?"
There should be a sticky thread on this subject with a big arrow pointing to it.

...........::shocked::

There is nothing scary about August Underground,gross yes not not scary in the slightest.