There's many tall and smooth towers of some kind of stone, surrounded by coral and bright bioluminescence. Though merpeople had a decent ability to see in the dark, it only worked up to a certain extent, and the light helped guide them all home.

"Hide behind me," Haruka says, stopping in the water and just about backing into Rin. "Just until we get to my place."

Judging from the dimness of the light, roughly matching the light for the surface about the ocean, Haruka knew that most people would be sleeping by now. Once Rin is in his room, he'll check in quickly with the others before returning.