For most of his life, Evan Dossey has generally avoided horror films. The genre makes him profoundly uncomfortable. This means he has enormous gaps in his cinematic knowledge. Each year, he asks friends and family which essential horror movies he needs to see in order to fill those gaps and spends the better part of October agonizing over them, tossing and turning over them … and writing about them. Once again, he’s sharing the month with those friends and family — letting them offer their own thoughts about the tales that terrify (or perhaps just titillate) them. This is our No Sleep October.

As an avid, borderline-obsessive horror film viewer for more than 35 years, I’ve seen … not every horror film ever made, but a good number of the more acclaimed, famous and infamous ones. And I say this without hesitation:

The Descent is the scariest film I’ve ever seen, an exhaustingly terrifying cinematic experience of my adult life.

The film starts with an event concerning terror of a different sort, where someone’s life changes in the blink of an eye and in one fell, almost unspeakably terrible swoop. A lesser film would make this moment far less shocking, give it far less weight. Instead, the explicit presentation, and the utter trauma it causes, sparks the start of one of the greatest horror films ever made — one that teaches Sarah (Shauna Macdonald) that no matter what she might think, she’s not at rock bottom … and it can certainly get worse.

At the same time, writer-director Neil Marshall also makes a statement to the audience that he’s not interested in making a modern, wink-wink horror film with loud, crashing music cues — where a monster pops out for a scare or a quick kill to elicit screams, and then laughter, from the audience. Marshall completely succeeds in creating an experience that, like a real horrific event, creeps inside your skin and seeps into your soul.

Marshall picks up the main narrative a year after this initial trauma, when Sarah is still trying to piece her life back together and her friends want to help. Juno (Natalie Mendoza) has taken the lead, a Type-A weekend warrior who has identified the source of their next adventure — spelunking. It’s just what Sarah needs, an adventure vacation with her best buds.

Marshall lulls the audience into comfort, setting most of Act I inside a cabin, introducing us to each character and giving us a glimpse into the group’s simmering dynamics. Everyone is concerned with Sarah, of course, but there are unresolved issues within the group — namely that Juno has in the past been a bit reckless and perhaps done something unseemly.

This confluence of events lead the women into hell’s hand-basket: the initial trauma, Juno’s alpha-dog determination to do something huge (which leads the group to realize, at a most inopportune time, that Juno has chosen to mislead her friends), and the group’s trust of someone whose judgment has proven to be questionable in the past.

Not only did Juno refuse to choose an easy-to-explore cave, she selected one that was only recently discovered and not yet explored — which means the first to survey the space means the glory of naming the cave after yourself. This means their original filed plan is a fraud, and a rescue party would be unable to find them were it to become necessary.

Even as the gang enters the caverns, Marshall’s hands are light — only oh-so-gently ratcheting up tension as the cave walls slowly close in on the group. And it’s after the moment that those walls are a little too close, an hour in, that the horror finally springs to life.

A sequence where one character becomes stuck activates a claustrophobe’s panic meter while a cave-in further raises the stakes — trapping the group, forcing Juno to admit her deception and sending the women into a full-on panic.

It’s here that The Descent hits the REM cycle for a nightmare, as the women discover their cave to be the home of a monstrous, flesh-eating group of humanoids. With skin like uncooked chicken and the eyes of a species that adapted to live in the dark (which is to say they’re blind but have keen hearing), the inhabitants are ruthless, animalistic killers who strike quickly and with the intensity of something choosing between its next meal and death.

The kills are brutal. Literal eviscerations, monsters devouring characters alive, throats ripped, veins stretched and snapped. Those who survive are forced to themselves become animals — willing to do unthinkable things to grotesque creatures looking to do even more unimaginable things to them.

It’s not just the creatures in The Descent that are scary — not just the jump scares, the mercilessness of the creatures or the length of time Marshall lingers on the eviscerations, although they all contribute. It’s the journey, and the sadness of the characters, that makes the horror all the more real.

The film’s seminal image, where Sarah falls into a large pool at least partially made of blood, represents a transformative moment in the film and character — the turning point where she taps into her inner animal and her rage extends past the creatures tormenting her.

It’s then that we are also forced to consider the full tragedy of it all; even if Sarah escapes the cave, the consequences of her choices will haunt her for years to come and PTSD will continue to bring her back to this event. Much like the tragedy Sarah experiences at the film’s outset, she will never truly escape the experience, even if she survives.

The Descent isn’t complicated in its plot, but Marshall executes that narrative so expertly that this becomes an experience that stays with you, right down to its dual endings. For my money, the bleaker Unrated original vision is a far better ending than the R-rated, Americanized happier ending.

The former represents Marshall’s defining vision for this film: In real life, there are no rolling credits to signify that you can move past the trauma of terror. It’s a lasting, life-altering event that forever reshapes those it touches.