It is possible that for some reason survivors of this race sought refuge in the abysmal spaces, one of whose entrances is on the islet where Throckmartin's party met its end.
"As for their persistence in these caverns--we know they possessed a high science.

They may have gone far in the mastery of certain universal forms of energy--especially that we call light.

They may have developed a civilization and a science far more advanced than ours.

What I call the Dweller may be one of the results of this science.

Larry--it may well be that this lost race is planning to emerge again upon earth's surface!" "And is sending out your Dweller as a messenger, a scientific dove from their Ark ?" I chose to overlook the banter in his question.
"Did you ever hear of the Chamats ?" I asked him.