Loss Brain arc
Kouhei's Group was led by Kouhei Arita into the cavern. Kouhei was hurt by his head. After that he started to kill his classmates. Yuki Sakuma escaped from his killing attempt. Once outside she inlisted Akira's Group to rescue her group. When Akira's Group arrived inside the cavern, they discovered more dead bodies and the culprit was none other than Kouhei himself. During that night water rains fell inside and caused flashflood. Both groups had to escape before the flood reach them.
The Limestone Cavern was mentioned again when Miina Isurugi (Fake) suggested the face on the walls were man-made.