It's a decent thriller, considering the microscopic budget and the mediocre actors, of whom I hardly remember two (Bruce Greenwood and Olivia Williams). You could say 'Below is a hit among B movies.
Firstly, the rather original plot draws attention to itself. Two big pluses of the script are the complete absence of dumb dialog between the crew members (which is what many more famous thrillers and disaster movies suffer from) and the complete absence of a love line, which seems unbelievable.
Secondly (which again is surprising), 'Below' has several pieces of good special effects, while the whole movie cost a couple of million bucks. The literacy of the director's use of funds is simply admirable.
Finally, thirdly, several times during the movie the viewer gets scared, and this allows us to recognize 'Below' as not the worst representative of the genre.