The Descent succeeds on many levels: as an action based character study, clever use of a claustrophobic setting, and on a deeper level an exploration of gender, trauma, and survival.
We begin with a prologue with Sarah leaving her friends after a festive weekend, then her husband and daughter are killed in a horrific car accident, reminiscent of The Changeling. A year later Sarah reunites with her friends for a weekend of cave exploration.
From that point on the story becomes one of survival. The women get lost then discover there are mutant creatures who fester in the cave. Already in a dire situation, it gets really brutal when these monsters strike with hints of Alien, yet it's also a human against nature story in the Jack London tradition.
The Descent is often compared to The Thing, in that it's all female characters facing awful creatures. The men in The Thing quickly come apart, they have been isolated together for months, and their fragile psyches are disintegrating. The women here are all close friends and have histories with each other. When they realize the extent of the threat the women work together, but also offer each other emotional support.
As a study of trauma The Descent relies less on shocks, but more on the aftermath of the shocks. When characters meet their deaths, it's never played for irony, they are excruciating as we the audience must find the courage to endure the movie. Life and death are treated with honesty and respect.
The look of The Descent is also memorable, the mix of yellows and blacks in the cave disorient perceptions and keep you on edge. Instead of jump scares, director Neil Marshall uses silence and darkness to evoke a terror that can strike at any moment.