Tunnel of Reflection (Blue Archive) (Pixiv Fanbox)

That summer, as their group was researching ancient ruins, Koharu accidentally activated a trap that flooded the tunnel where she's in! As she was struck without warning, she almost lost her consciousness when the water submerged her. Thankfully Sensei was at the other end of the tunnel and able to get her. But now both of them are trapped! Will they be able to escape? (spoiler: they did, with some unexpected help :D)