It was the ever receding wall of darkness.
He did not know how long they traversed the chill sewer in this fashion.
In time, however, the water got deeper; rats began to scurry along the sides of the circle or to swim frantically on in front of the disturbers.

The smells were sickening, overpowering.

Only excitement, curiosity, youth--whatever you may care to term it-kept him up and going.

The everlasting glory of youth never ends until old age has provided the surfeit of knowledge; the strife to see ahead, to find out what is to be, to know,--that is youth.

Youth dies when curiosity ends.
The emotion is even stronger than the dread of what may lie beyond in the pallid sea of uncertainty.
His bones were chilled and creaking with fatigue.