[Potential treasure is right. Poking through the ruins with a careful eye might have shown a hint of just how wealthy or important the patrons of this place might have been, for the stone of the statue and door and parts of the walls and floor are made of something different from the rough natural cavern that intersects at the ravine, or that tumbles into the hallway where the cave-in occurred. This stone is fine-grained and lustrous. Worked smooth, polished, though age has covered some of that fine work with calcification and dust. But the statue and its alabaster chalice stand testament to the importance this place or its guests might once have held.]

Let's think a second before we spend time doing that. What's that on your glove, what was in that cup? Do you think it's just mud? What if it was wine or... something else? The writing on the cup looks a lot like what's on the door. What if instead of taking the key out of the cup, you put the key in it?

[Which is weird and almost magical, but he's seen magic now. Some places, it actually exists. Other places, things like devil fruit can look like magic to the untrained eye. Though if this is a devil fruit effect, that would mean the user is still alive somewhere...]