Let me start off by saying that I'm no 'film snob.' I love cheesy horror movies and have quite a lot of 'found footage' films among my DVD collection. Since the sub-genre was revived by the 'Blair Witch Project' film-makers have tried copying its success by shooting a low budget film as if it was actually shot by the characters in the story.
Most 'found footage' films don't live up to the stand-outs like the 'Blair Witch Project' and 'Paranormal Activity,' but at least they do enough to be worth a watch. 'Underground' does not. I try to find something good in even the worst films, but it was hard to say anything positive in this case. I guess the actresses' performances were reasonable, i.e.naturalistic and therefore befitting of the film-style.
Otherwise, steer well clear. It's even longer than the average film and the first half hour is basically 'getting to know the characters.' Only the characters aren't that interesting. It's like a stranger asked you to sit down and watch their holiday video. You'd have to be really bored to oblige.
The plot is about a hen night where the women get lost in a tunnel system and - naturally - fall foul of something nasty that dwells down there. It's a decent enough premise, but nothing really happens. Once they do get lost of have long - and I mean looooooong - drawn out shots of the women sitting around in the darkness. If you haven't already turned off there's about five minutes worth of 'supernatural threat' towards the end. Then it ends. Only it doesn't. It sort of ends then starts up again for no real reason (or rather to offer one last 'scare' which isn't scary).
And that's its major problem - it's just not scary. I'd say if you're in the mood for horror/found footage then any other movie in those genres would be better than this. In fact... any movie in any genre would probably be better than this.