into all the cross-headings and chambers you see. But, above all," he
continued, "don't fail to leave a light here at a shaft, and be
careful that you never pass out of sight of it."

Canfield hastened away, climbing the ladders two rungs at a time, and
soon disappeared into the little dot of light at the top.

The two boys searched patiently for the boat for a long time, but did
not succeed in discovering it. At last, Will suggested that it might
be in the mule stable and thither they went.

The boat was there, in excellent condition, and the boys soon had it
swinging to and fro on the surface of the water which now lay several
feet deep in the main gangway.

"Jerusalem!" exclaimed George, taking the depth of the water with an
oar, "if the water is four feet deep here, how deep must it be at the
middle of the dip?"

"About forty rods, I should think!" exaggerated Will.

The boys left a large searchlight at the shaft so situated that it
looked straight down the passage they proposed following, and started
away in the boat. The flashlights illuminated only a small portion of
the underground place, but the boys could see some distance straight
ahead.

Once they ceased rowing to listen, believing that they had heard calls
from the darkness beyond. The sound was not repeated, and they were
about to proceed when a sound which brought all their nervous energy