I haven't seen that whats it about
It is in "found-footage" style which is usually a clear for me. But it's actually surprisingly coherent, and develops characters somewhat appropriately while getting into the real action within 10 minutes. Plus, it's virtually all practical effects. It's about a group of people who have a background in mythology, etc. that explore the catacombs below Paris that contain the remains of like 6 million people. They get lost, and... well, you see where this is going.
There are the typical "first-person horror" trappings from time to time but fukk, it's got some great scares. And that main girl is . Definitely peep it if you can, but just don't expect any Kubrickian plot and enjoy the ride.