And long did I puzzle to find out why the water stood in some craters, while others, within a mile of them perhaps, were perfectly dry.

That I never found out for myself.

But learned men tell me that the ashes which fall back into the crater, if the bottom of it be wet from rain, will sometimes "set" (as it is called) into a hard cement; and so make the bottom of the great bowl waterproof, as if it were made of earthenware.
But what gives the craters this cup-shape at first?
Think--While the steam and stones are being blown out, the crater is an open funnel, with more or less upright walls inside.

As the steam grows weaker, fewer and fewer stones fall outside, and more and more fall back again inside.

At last they quite choke up the bottom of the great round hole.