“There is, I believe, nothing in the lunar world that would not seem strange to our eyes. To understand just what this picture means you should imagine yourself floating in an airship at an immense height above the surface of the moon. The _Mare Serenitatis_ you will recognize as the great oval plain occupying the upper left-hand part of the photograph. It is entirely encircled by mountains except in three places—at its eastern end, where a broad strait opens between the Apennines on the south and the Caucasus on the north, leading into the _Mare Imbrium_; on the northwest, where another strait opens into the _Lacus Somniorum_, the ‘Lake of the Sleepers,’ or ‘The Dreamers,’ and on the southwest, where a third strait with a conspicuous crater in its center leads into the _Mare Tranquillitatis_. The _Mare Serenitatis_ is 430 miles long and nearly as broad, and covers an area of about 125,000 square miles. A great many details are visible on its floor. Even if it were covered with water we might see these, for, as you have probably heard, the bottom of deep lakes is visible when one looks down upon them from a great height. The surface of water, however, at certain angles of view and of illumination, would produce flashes and glares of light which are never seen on this vast lunar plain.” “Oh, but it _must_ once have been a sea,” said my friend, poring over the photograph. “I cannot give up that idea. It gives the interest of life to the moon, if not now at least in the past.”