That passage must have been blocked up.

As for the great opening in the side of the cave, the rocks have fallen in there--that is easy enough to see.

Yes, men made this cave and filled it with water, and if the water were high enough to cover the handle of that machine, as it was when I struck it, it must also have been high enough to cover up this stone mound.

The lake was intended to cover and hide that mound.

And then, to make the hiding of it doubly sure, the men who built all this totally covered up the lake so that nobody would know it was here.