The searchlight was kept at work, flashing hither and thither, and every new object of interest was carefully studied.
Soon the topography of the Sunken Isthmus began to change. The rocky hills sloped gradually away into a plain.
Here the Diver descended very close to the bed of the sea and Wade outlined quite distinctly the original coast.
The sinking of the earth’s crust, which had resulted in this submersion of the isthmus, was not to be easily explained. Some internal revolution was very likely responsible for it.
“Now,” cried Wade, after some careful study, “let us change our course to the south; I would like to know the exact width of the isthmus.”
The Diver’s course was changed accordingly. For four hours it sailed over the deep-sea plain. Then indications were plainly seen which told that this was the southern extremity of the isthmus.
“Fifty miles,” announced Frank, as he consulted the gauge; “that is the breadth of the isthmus at this point.”
“It is probably the average breadth,” said Wade, “although it is not impossible that it may have been wider in some other localities.”
The course was now changed to the east. It was not a great distance to the Cuban coast.
Half a day’s steady deep-sea sailing showed the usual signs of the surface, and Frank brought the Diver to a stop just over a jagged reef of coral.