The reservoir is the real star. Lee Sang-min shoots it in fog, rain and pitch darkness, and Korean critics praised the "damp realism" that lets the horror "seep into the body". Some scenes were filmed at the real Salmokji Reservoir, and you can feel the cold.
The best idea is the road-view cameras. The crew records the area, then finds people and shapes in the footage that nobody remembers seeing. Scenes where Su-in watches a colleague's live camera feed from a distance are some of the creepiest in the film.
Kim Hye-yoon gives a restrained, serious performance, and Korean media dubbed her a new "horror queen". Lee Jong-won and Oh Dong-min also stand out. But most of the crew are thin characters who exist to disappear, and international critics noted that the scares, while effective, are familiar.
It is not a reinvention of the genre. It is a well-made, classic haunted-place horror film with great atmosphere and plenty of jump scares, which explains why audiences flocked to it.
