For some days the Dart stood straight out to sea. Frank had made his course by the best of the submarine charts.
He had now reached what he believed to be the entrance to the great valley under the sea.
This was at the beginning of the southeast branch of the Gulf Stream. The submarine course would extend to within a few hundred miles of the Azores and then southerly, finally terminating at Bermuda.
All this vast space was a mighty depression, known as the Great Valley.
It has ever been a mystery to sailors and geographers from early times.
Ancient chroniclers speak of an old-time continent and nation of people due west from the coast of Spain.
As this continent does not exist to-day, it has been believed that it has sunk by some mighty process of nature many centuries ago.
There are plenty of mythical tales of the sunken world and its wonders now lying under the sea.
That the keels of our modern ocean greyhounds may daily pass over a sunken world is by no means improbable.
Perhaps some day our own American continent may be relegated to a like fate.