A recreational diver who spotted old timbers in Germany's Lake Constance may have discovered the wreckage of a boat built in the 1400s. The unexpected discovery is giving archaeologists a rare look at medieval trade and travel on one of central Europe's largest lakes. What happened? Near Lindau, Germany, a diver noticed wood jutting from the lakebed. He reported the find to Professor Tobias Pflederer of the Bavarian Society for Underwater Archae…