Passages ran in every direction, crossing and recrossing, and it seemed wonderful that the boy Sacho knew just which way to go.
But he never hesitated an instant.

Trot looked carefully to see if there were any marks to guide him, but every wall was of plain, polished marble, and every turning looked just like all the others.
Suddenly Sacho stopped short.

They were now in a broader passage, but as they gathered around their conductor they found further advance blocked.

Solid walls faced them, and here the corridor seemed to end.
"Enter!" said a clear voice.
"But we can't!" protested Trot.
"Swim straight ahead," whispered the boy in soft tones.

"There is no real barrier before you.