December 7, 1941, was indeed a day that will live in infamy -- but Japan wasn't the only Axis power to attack the United States in the opening days of World War II.

For the first few months of 1942, Nazi U-boats patrolled the waters along the eastern seaboard and the Gulf of Mexico, wreaking havoc in a now-forgotten campaign that sank 609 ships in U.S. waters.

It all started in December 1941, when Vizeadmiral Karl Doenitz (alternately Dönitz), commander of the Nazi submarine fleet, got an early Christmas present from Der Fuehrer. His orders were to take a half-dozen U-boats and head for a carnival-style duck hunt off America's Atlantic coast and in the Gulf of Mexico, sinking any ship that floated.

"The first U-boat, U-123, got underway Christmas Eve or thereabouts, reached American waters in January, sailed very near New York -- the captain saw the skyline -- and sank a ship," says Dr. Allen Cronenberg, a retired history professor at Auburn University and an expert in the field. "That's basically when the war on the American coast got underway."

Keep reading to find out when the Gulf of Mexico was "the deadliest place on Earth" and where on the U.S. coast you can still visit wreckage of Nazi U-boats that were sunk once America slowly realized its waters were under siege.

For months before America entered the war, U.S. ships had been carrying vital weapons and supplies to England. The cargo ships sailed alone -- slowly and well lit -- with no wartime precautions. Along the U.S. coast, cities kept the lights burning brightly all night, making cargo vessels stand out as fat, juicy silhouettes in the sights of the Nazi U-boats.

U-Boats Sank Dozens of Ships in U.S. Waters
On Jan. 14, 1942, an oil tanker bound for Great Britain was sunk by U-123 within sight of Long Island. Four days later, two more tankers sank in the same area. The next day, a pair of steamships went down. On Jan. 22, a freighter was sunk off Cape Hatteras, N.C. By the end of the month, German subs had sent 14 ships to the ocean floor along the eastern seaboard of the U.S.

It was just the beginning. The U.S. coastal assault was so effective, Hitler gave Doenitz carte blanche -- and additional U-boats, launched in waves.

With no deterrent, the German U-boats found U.S. shipping liners to be easy pickings. Seventeen cargo vessels were sunk along the eastern seaboard in February, a staggering 35 in March and another 28 in April.

In May, U.S. naval vessels, recalled from other theaters of war, began escorting freighters, traveling in lights-out convoys. Coastal cities enforced blackout conditions.

In response, the Nazis shifted their operations to the Gulf of Mexico, which was still lightly defended. By the end of the month they'd sunk another 27 freighters in the same waters that BP is currently befouling.

The Gulf of Mexico Was "The Deadliest Place on Earth"
Says Dr. Cronenberg, "From May through July of 1942, the Gulf of Mexico around to the east coast of Florida up to Jacksonville was the deadliest place on Earth for shipping." From May through September, U-boats sank 58 ships in the Gulf of Mexico alone, accounting for more tonnage than any other period of the war to date in all other theaters combined.

By 1943, the industrial might of the U.S. began building ships faster than the Germans could sink them, and the Navy had developed more sophisticated defensive capabilities.

Small aircraft carriers put anti-sub planes in the game. Radar technology improved. The waters of the Gulf and Caribbean were teeming with anti-sub vessels -- even author Ernest Hemingway got in on the act, arming his fishing boat the Pilar to the teeth with .50-caliber machine guns, bazookas and grenades, and setting out on patrols in the Florida Straits with his sons and friends.

The previously impervious U-boat fleet suddenly found itself in trouble.

Nazi Submariners Spent Time in U.S. Towns and Ports
Once the U.S. started blowing up U-boats, rumors abounded that bodies of crew members were found floating among the wreckage clad in civilian clothes and carrying American I.D. cards and greenback dollars.

Receipts from inland laundries and bread from local bakeries were supposedly recovered as well, showing the submariners were no strangers to American ports and towns.

After losing seven U-boats, Doenitz called off the American campaign and returned the fleet to Europe.

But the seven Nazi U-boats that were sunk off American coasts still lie where they settled. One is in less than 100 feet of water off the North Carolina coast. Another sits 45 miles south of the Mississippi River Delta. Some are preserved as landmarks, watery graves and historical markers. At least one is rumored to serve as a secret dive spot, only talked about in whispers.