If all you want is a spooky movie with monsters and atmospheric settings, this is as good as any. If you expect anything like a logical story, well, maybe not so good. The film depends on a common misconception; every big city has endless tunnels beneath and they are all connected as well as lost and forgotten. And this is nonsense. Unlike Paris which has quarries beneath it, most cities do not have anything like a connected maze of tunnels. And most tunnels, even abandoned subway stations, are used for something and are well documented.
This film, like others of its type, substitutes basements or even abandoned factories with their windows covered for underground areas. One look at the brickwork of these phony tunnels is enough to reveal the sets are ordinary locations dressed as spooky tunnels. The huge size of many of the locations should tip off the viewer that they are seeing nothing more mysterious than an old warehouse.
And our explorers are totally unprepared for their adventure. The have no hardhats, no first aid kit, no proper clothing, nothing but a few dinky flashlights. If not for their "guide" somehow toting 5 sets of rubber waders in his tiny backpack they would never have made it through a flooded tunnel. We are further told that scrawny teens can climb 50 feet, hand over hand, on a thin rope. And thats up and down. Small wonder the actual climbing is not shown! And of course the abandoned tunnels are full of improbable characters who survive on.....what....rats? If you like to suspend your disbelief totally, you will probably enjoy this film. But if, like me, obvious plot holes spoil the experience you may wish you watched something else.