Spelunking. Even the word sounds like something you couldn't pay me to do, let alone invite me to do as part of a pleasant weekend away. The potential for danger is right there in those three, ugly-sounding syllables. It's that danger which provides Neil Marshall's 2005 nightmare The Descent – the scariest movie ever made – with its horrifying impetus. This is a film in which the opening credits have barely faded out before a car crash culminates in a bloke and his daughter each getting a copper pipe through the face. Marshall doesn't give a damn about your comfort.

For those who haven't seen it, the film follows a group of half a dozen women as they explore a system of caves – for fun, apparently. It has been a year since Sarah (Shauna Macdonald) lost her family in the aforementioned accident and her buddies want to cheer her up. Among the group is the warm, dependable Beth (Alex Reid), de facto leader Juno (Natalie Mendoza) and the wildcard thrill-seeker Holly (Nora-Jane Noone). Marshall's opening scenes are a masterclass in economical writing, with every line of dialogue gently and subtly illuminating the group dynamic, while giving us every bit of info we need about who these women are.

Then, they head down into the cave.

I consider myself a horror fan and I like to think that really getting under my skin takes a bit of doing. With that said, I had only seen The Descent once prior to writing this piece, for which I forced myself to watch it again. My reasons for eschewing a rewatch until now were twofold. First and foremost, I remembered being absolutely terrified out of my skin for hours and, secondly, I was worried it would have lost its power on second viewing. I was right on the first count.

The Descent is scary before the characters delve underground – there's a killer dream sequence jump scare, for example – but it kicks up a notch once the walls close in and things get dangerous. There are tunnel collapses, revelations about botched safety measures and increasing paranoia among the women. Then, just as things surely can't get any more tense, cave-dwelling beasts start tearing people's throats out. Marshall, you evil bastard.

This is a movie of pure intensity. Claustrophobic horror always has an extra frisson of danger, whether it's the final moments of underrated British flick The Borderlands or the French catacomb tale As Above So Below, but Marshall takes it to a fine art. Every mishap is another twist of the suspense knife and, thanks to that excellent early character work, every blood-soaked death is keenly felt on both sides of the camera.

The Descent also deserves credit for its refusal to fall into the obvious trap of cliché, which hovers over the characters. A lesser movie would draw a web of betrayals and tensions between these women, in order to unveil it for a blast of narrative spice throughout the story. There is one rather soapy secret at play in The Descent, but Marshall almost throws it away and has it usurped by something entirely different. By the time the information is out in the open, it's effectively irrelevant.

Marshall rightly realises that the tension here comes not from seeing these women come apart as friends, but from seeing the ways in which they interact while fighting for their lives. These aren't schematic horror protagonists wandering into the clearly haunted basement because the narrative requires it. They're rounded, believable women who take the actions they do because it's in line with how their characters would react if this were to play out in real life. Some are selfless, some prioritise their own life over others and some are simply better equipped for a Darwinian struggle than others.

It would be easy to pick out Shauna Macdonald as the standout here – and she's outstanding as a Final Girl for the ages – but that does a disservice to how well this cast works as an ensemble. Every woman has their part to play and they all do it to perfection. Mendoza is an utter badass, as defiant as she is crippled by a hero complex, while Reid as heart-breakingly brilliant as the supportive buddy dragged along for the dangerous ride.

Then there's the ending. The real ending, mind. Not the American one. The Descent has a deliciously bleak final fake-out, emphasising the futility of fighting for survival in a system that has doomed you to failure, but that wasn't palatable for American test audiences. Stateside, the movie ended a minute or so earlier and preserved a glimmer of hope. Marshall doesn't acknowledge this ending as a legitimate one, and neither should you.

The Descent has endured as one of the scariest movies of all time because of its rigidity of vision. It was a female-focused horror film before Hollywood decided there was money in depicting the other 50% of the population, but it's a movie that wears that lightly. Simply, it's a lean and mean tale of terror that weaponises fears of being trapped in the wilderness with which we can all identify.

And there's throat-ripping ghouls too. Don't forget those guys.

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