Associate Professor Akira Takatsuki's Conjecture, Vol. 7 (light novel)

A female Seiwa student comes to Professor Takatsuki and Naoya looking for advice. She tells them about going with her club friends to a building where the elevator supposedly transports whoever rides it to another world, and the next day, one of their group vanished without a trace. Next, the pair receive a request from afar and head deep into the mountains of Tochigi. They find themselves in the middle of a construction site where mysterious events continue to occur as the builders try to fill in the marshland it sits atop. However, legend has it that the marsh is home to "Nushi-sama"...