Their way was, however, speedily blocked by a number of rocks piled over the entrance.
"This was evidently the original entrance to these caves," Ralph said, "but it was covered up when the rocks came down from above.

That would account for the place not being known to the coast-guards.

I thought the passage we came in by looked as if it had been enlarged by the hand of man.

No doubt it was originally a small hole, and when the entrance was blocked the men who made up their minds to establish a still here thought that it would be the best way to enlarge that and to leave the original entrance blocked.
"Well, it's evident we must take Captain O'Connor and the wounded out by the small entrance.

It would be a tremendous business to clear those great rocks away." Captain O'Connor and the two wounded men were with great difficulty taken through the narrow passage.