The Land that Time Forgot by Edgar Rice Burroughs

Off the coast of Greenland, a man finds a floating thermos bottle. Wedged tightly inside is a sheaf of papers covered with minute handwriting. As he begins to read, a fantastic tale begins to unwind. The writer, on his way to a WWI battlefield was shipwrecked and his entire regiment except for a woman and his faithful dog are killed. The three are rescued by a passing British tug, but fall prey to the schemes of a German spy aboard. They are then captured by the crew of a German U-boat.