"How can any one tell how long ago the last glacial epoch closed ?" "Figure out how far the melting line traveled from south to north," said Fenton, "then figure that the glaciers receded at the rate of only twelve feet every hundred years, and you'll know something about it." "Come on!" cried Frank, "let's get in there and find their Croton system.

I'm so thirsty my throat sizzles.

Come on!" Nestor, closely followed by the others, led the way into the south chamber, called, for convenience, "Chamber B" on the rough map made later on.

The place was damp and cold, and a current of air came from the southwest corner, indicating an opening there.
After clearing away a heap of rocks and loose sand, which might once have been rock, the boys found an opening which had been, apparently, closed for a long period of time.

When finally cleared, after an hour of hard work, the opening from which the current of air had come was discovered to be a door like arch in the west wall of the main chamber.
The electric flashlight, however, when introduced into the opening, showed a narrow passage beyond the opening instead of a square room.
This tunnel-like passage was not far from six feet in width and about that in height.