Virgil explains that the waters of this stream are formed by the tears of the unhappy, which are plentiful enough to feed the four mighty rivers of Hades! While following the banks of this torrent, Dante questions why they have not yet encountered the other two rivers which fall into the pit; and discovers that, although they have been travelling in a circle, they have not by far completed one whole round of the gigantic funnel, but have stepped down from one ledge to the other after walking only a short distance around each circumference.
_Canto XV._ The high banks of the stream of tears protect our travellers from the burning sands and the rain of fire, until they encounter a procession of souls, each one of which stares fixedly at them.

One of these recognizes Dante, who in his turn is amazed to find there his old school-master Ser Brunetto, whom he accompanies on his way, after he learns he and his fellow-sufferers are not allowed to stop, under penalty of lying a hundred years without fanning themselves beneath the rain of fire.

Walking by his former pupil's side, Brunetto in his turn questions Dante and learns how and why he has come down here, ere he predicts that in spite of persecutions the poet will ultimately attain great fame.
_Canto XVI._ Reaching a spot where the stream they are following suddenly thunders down into the eighth circle, Dante beholds three spirits running toward him, whirling round one another "in one restless wheel," while loudly exclaiming his garb denotes he is their fellow countryman! Gazing into their fire-scarred faces, Dante learns these are three powerful Guelfs; and when they crave tidings of their native city, he tells them all that has recently occurred there.
Before vanishing these spirits piteously implore him to speak of them to mortals on his return to earth, and leave Dante and Virgil to follow the stream to the verge of the abyss.

There Virgil loosens the rope knotted around Dante's waist, and, casting one end of it down into the abyss, intimates that what he is awaiting will soon appear.

A moment later a monster rises from the depths, climbing hand over hand up the rope.
_Canto XVII._ This monster is Geryon, the personification of fraud, and therefore a mixture of man, beast, and serpent.