Out of the Depths by Robert Ames Bennet — Read

A day of anxiety, only partly relieved by those tiny flashes of light so far, far down in the awful depths; then the long night of ceaseless watching. Neither Genevieve nor Isobel had been able to sleep during those hours when no flash signaled up to them from the abysmal darkness.

Then at last, a full hour after dawn on the mesa top, the down-peering wife had caught the flash that told of the renewal of the exploration. As throughout the previous day, Gowan brought the ladies food and whatever else they needed. Only the needs of the baby had power to draw its mother away from the canyon edge. Isobel moved always along the giddy verge wherever she could cling to it, following the unseen workers in the depths.