Ten thousand, twenty thousand, a hundred thousand years ago--the last more likely.
"All these islets, Walter, are squared, and their shores are frowning seawalls of gigantic basalt blocks hewn and put in place by the hands of ancient man.

Each inner water-front is faced with a terrace of those basalt blocks which stand out six feet above the shallow canals that meander between them.

On the islets behind these walls are time-shattered fortresses, palaces, terraces, pyramids; immense courtyards strewn with ruins--and all so old that they seem to wither the eyes of those who look on them.
"There has been a great subsidence.

You can stand out of Metalanim harbour for three miles and look down upon the tops of similar monolithic structures and walls twenty feet below you in the water.
"And all about, strung on their canals, are the bulwarked islets with their enigmatic walls peering through the dense growths of mangroves--dead, deserted for incalculable ages; shunned by those who live near.
"You as a botanist are familiar with the evidence that a vast shadowy continent existed in the Pacific--a continent that was not rent asunder by volcanic forces as was that legendary one of Atlantis in the Eastern Ocean.[1] My work in Java, in Papua, and in the Ladrones had set my mind upon this Pacific lost land.

Just as the Azores are believed to be the last high peaks of Atlantis, so hints came to me steadily that Ponape and Lele and their basalt bulwarked islets were the last points of the slowly sunken western land clinging still to the sunlight, and had been the last refuge and sacred places of the rulers of that race which had lost their immemorial home under the rising waters of the Pacific.
"I believed that under these ruins I might find the evidence that I sought.
"My--my wife and I had talked before we were married of making this our great work.