Their chance search was rewarded by something they had not seen in the sea-beds of the other hemisphere.
These depressions were far deeper than the others, evidently many thousands of feet below the average surface, but the sun's rays were blazing full into this one, and, dotted round its slopes at varying elevations, they made out little patches which seemed to differ from the general surface.
"I wonder if those are the remains of cities," said Zaidie.

"Isn't it possible that the old peoples of the moon might have built their cities along the seas just as we do, and that their descendants may have followed the waters as they retreated, I mean as they either dried up or disappeared into the centre ?" "Very probable indeed, dearest of philosophers," he said, picking her up with one arm and kissing the smiling lips which had just uttered this most reasonable deduction.

"Now we'll go down and see." He diminished the vertically repulsive force a little, and the _Astronef_ dropped slantingly towards the bed of what might once have been the Pacific of the Moon.
When they were within about a couple of thousand feet of the surface it became perfectly plain that Zaidie was correct in her hypothesis.

The vast sea floor was thickly strewn with the ruins of countless cities and towns, which had been inhabited by an equally countless series of generations of men and women, who had perhaps lived and loved in the days when our own world was a glowing mass of molten rock, surrounded by the envelope of vapours which has since condensed to form our oceans.
They dropped still lower and ran diagonally across the ocean-bed, and as they did so Zaidie's proposition was more and more completely confirmed, for they saw that the towns and cities which stood highest were the most dilapidated, and that the buildings had evidently been torn and crumbled away by the action of wind and water, snow and ice.
The nearer they approached to the central and deepest depression, the better preserved and the simpler the buildings became, until down in the lowest depths they found a collection of low-built square edifices, scarcely better than huts, which had clustered round the little lake into which, ages before, the ocean had dwindled.

But where the lake had been there was now only a shallow depression covered with grey sand and brown rock.
Into this they descended and touched the lunar surface for the last time.