It's a fantasy role-playing game.

DM: You are Percival de Rolo, a human gunslinger . You are the member of the famous group of adventurers. Previously, They learn that their movement and ranged attacks will be severely impeded, Percy's guns won't work at all, and Keyleth can't speak to cast spells. The party finish their underwater session and eat a Heroes' Feast (or, in this case, a Heroes' Midnight Snack).
DM: Looking in your vicinity, you see clusters of ruins, you see broken pieces of very, very old stonework. And looking at them, they are scattered throughout the vicinity. Singular chunks of structures. Let's see. Percival, go ahead and make an intelligence check.
Player: That'll do. 20.
DM: Okay. Looking to the left of you as you pass by this one structure, you can see the chunk of wall that's visible, it is thick enough and tall enough where it looks like it's a sliver of the base of a larger structure that was designed for far larger creatures. The sense of the erosion and the relatively smooth edges of the various broken parts to it tell you that it's been there for a very long time.
Player: This was for a larger creature, like, giants?
DM: Giants or larger. It's hard to really tell. This is not a human-sized--
Player: Did these ruins crumble, or did they sink?
DM: They appear to have crumbled. The structure in which they're placed has intentional placement within here.
Player: And is the stone-- does the stone feel natural, or does it feel like stone that would be underwater? Does it look like the parts of it that have been underwater are doing well, or does it look like it's eroding as something that should've been above the surface?
DM: It looks like they're eroding as expected for something that's been there for a very long time.
Player: And there's no writing visible.
DM: No, no. They're almost extremely smooth. Just been there for so long and hit with various blasts of water current, sand, dust, and other things for such a long period of time that it's smoothed the entirety of the wall structure.
