[RedDevil]

I've read that even ancient stone coffins can emerge from the soil after an earthquake.
In China they put the dead in stone coffins and sank them down in a grave pit filled with water, then put earth over it to fill the pit and drive away the water.

But while the coffins are heavier than their volume in water, they are lighter than the same volume in mud, and only the weight of the soil on top of them prevents them from floating up.

Until something disturbs the waterlogged soil, like an earthquake. The soil becomes fluid and streams down past the sides of the coffin while that moves up.

We don't know what went up there, but I would not bet on it being filled with air. It could do the same if it was filled with water or oil or whatever substance, as long as it is less dense than the mud around it.