British General John Burgoyne must have been bitterly disappointed one day in July 1777 in the upper Hudson Valley — the day his army, hot in pursuit of the Americans they had just driven from Fort Ticonderoga, ran into a lake that wasn’t supposed to exist. This part of upstate New York had already been thoroughly explored and mapped, yet the Redcoats, confident of speedily overtaking and finishing off the American force, suddenly found themselves blocked by a brand-new body of water where dry forest and field was supposed to provide swift passage. The British must have soon ascertained, as...