Looking closer ...for size perspective ...could that be a pontoon boat in the bottom right of the photo. I've looked at it a bit and don't believe it is but it sure resembles one.
edit to add:
I thought the pontoon was sitting on a rock at an angle, marooned there. Now I see it most likely is beached there and came in via that small inlet. It is in the water after all. LOL Huge cavern. Love it.
While I find it amazing and would likely explore it myself, I'd be mighty worried about how easily and often the ceiling peals away a slate of rock. I'd also compare the color of the fallen rock to the color of the ceiling to try and figure how long its been since it fell.