How it could have been formed, no one can tell.

Even wilder guesses than yours, Ardan, have been hazarded on the subject.

All we can state positively at present regarding this wonderful formation, is what I have just recorded in my note-book: the _Valley of the Alps_ is about 5 mile wide and 70 or 80 long: it is remarkably flat and free from _debris_, though the mountains on each side rise like walls to the height of at least 10,000 feet .-- Over the whole surface of our Earth I know of no natural phenomenon that can be at all compared with it." "Another wonder almost in front of us!" cried Ardan.

"I see a vast lake black as pitch and round as a crater; it is surrounded by such lofty mountains that their shadows reach clear across, rendering the interior quite invisible!" "That's _Plato_;" said M'Nicholl; "I know it well; it's the darkest spot on the Moon: many a night I gazed at it from my little observatory in Broad Street, Philadelphia." "Right, Captain," said Barbican; "the crater _Plato_, is, indeed, generally considered the blackest spot on the Moon, but I am inclined to consider the spots _Grimaldi_ and _Riccioli_ on the extreme eastern edge to be somewhat darker.

If you take my glass, Ardan, which is of somewhat greater power than yours, you will distinctly see the bottom of the crater.