Possible Entrance Under Water

So, I found this cave with a weird symbol on the ground and a possible entrance,
https://gyazo.com/90c4feb0c3f92085f0e7082c3daef9f4
https://gyazo.com/2e5ac8ac41be8a1fac9c76de34d5afc5
I know how to get inside the iron door, and this is what was inside:
https://gyazo.com/5bb4fe470b58545bf881b48c82e1e43e
https://gyazo.com/e08609e186e1342ca6fd34c606016286

We already tried activating all pressure plates, do you guys have any idea of how to open it, or if it even it an entrance?

 

what can you see on the other side of the door?

 

Just a cave with a button that opens the door and water on the floor

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And a camp that look like gerts

 

Looks like a loot chest spawns there