reflect, and this is the theory he evolved--

Some one, long ago, had discovered valuable minerals in the volcanic
rock. Mining operations were in full blast when the extinct volcano
took its revenge upon the human ants gnawing at its vitals and
smothered them by a deadly outpouring of carbonic acid gas, the
bottled-up poison of the ages. A horde of pigs, running wild over the
island--placed there, no doubt, by Chinese fishers--had met the same
fate whilst intent on dreadful orgy.

Then there came a European, who knew how the anhydrate gas, being
heavier than the surrounding air, settled like water in that terrible
hollow. He, too, had striven to wrest the treasure from the stone by
driving a tunnel into the cliff. He had partly succeeded and had gone
away, perhaps to obtain help, after crudely registering his knowledge
on the lid of a tin canister. This, again, probably fell into the hands
of another man, who, curious but unconvinced, caused himself to be set
ashore on this desolate spot, with a few inadequate stores. Possibly he
had arranged to be taken off within a fixed time.

But a sampan, laden with Dyak pirates, came first, and the intrepid
explorer's bones rested near the well, whilst his head had gone to
decorate the hut of some fierce village chief. The murderers, after
burying their own dead--for the white man fought hard, witness the
empty cartridges--searched the island. Some of them, ignorantly
inquisitive, descended into the hollow. They remained there. The
others, superstitious barbarians, fled for their lives, embarking so
hastily that they took from the cave neither tools nor oil, though they
would greatly prize these articles.