Environment : Unlocking the Secrets of 1940 Nazi Shipwreck : Norway is using a new technique to map oil aboard the destroyer Bluecher and assess pollution risks.

In the shivering dawn of April 9, 1940, Adolf Hitler’s newest, most formidable destroyer cut confidently through the frigid waters of the Oslo fiord, leading the Nazi invasion of Norway. The “unsinkable” Bluecher expected little resistance.

But at 5:19 that morning, as the Bluecher slowly maneuvered through the narrowest part of the fiord, a barrage of artillery fire thundered from a small hilltop fortress on a nearby island. Two ungodly explosions then pummeled the 680-foot warship, setting it ablaze and triggering its own stores of ammunition.

Barely two hours later, the pride of Hitler’s fleet sank to the bottom of the fiord, taking hundreds of German servicemen to a cold, watery grave.

Now, 53 years later, from its muddy resting place, the Bluecher is posing a new threat to Oslo, and Norwegians are relying on a combination of luck and ingenuity to head off disaster.

Like thousands of World War II shipwrecks littering the waters of the world, the Bluecher has now corroded nearly to the point of breaking apart. And with an estimated 1,200 to 1,500 tons of fuel still on board the destroyer, an ecological disaster looms should the oil spill into the slender fiord.

“There’s continuous leaking already,” said Sverre Sivertsen of the State Pollution Control Authority in Oslo. Tiny holes in the Bluecher’s hull routinely bleed fuel into the fiord.

The relatively minor leaks took an alarming turn for the worse around Christmas, when one of the Bluecher’s 182 connecting fuel tanks rusted through, disgorging 60 tons of oil. A 1989 investigation concluded that it could take another 20 to 40 years for complete breakup of the Bluecher, but the Norwegians feel they have no time to waste now.

“It’s a very fragile, very rich ecosystem around the fiord,” Sivertsen said, “and it’s also a major recreational area.”

Visitors already complain about the acrid smell and filmy water around the spot where the Bluecher lies upside-down on the seabed 300 feet below, near the picturesque village of Drobak about 20 miles south of Oslo.

Given the fresh urgency since the December spill, Norway is pioneering new technology to pinpoint where the oil is on the Bluecher and to assess pollution risks.

Without such mapping, pumping out the oil would be next to impossible, since no one is sure how much oil is stored in each of the tanks, or precisely where the tanks lie in the corroded wreckage.

Because the Bluecher is a grave for an estimated 600 to 1,000 German servicemen, disturbance of the wreckage must be kept to a minimum, and trying to raise or blow up the shipwreck is out of the question.

So, over the summer, a mini-submarine is scheduled to probe the sunken ship’s hull with a new sonar device for finding oil and evaluating the pollution potential.

“There’s a danger that we could have a breakup,” said Sivertsen. “We are working at the extremes of technology here.”

The U.S. Coast Guard and North Atlantic Treaty Organization already have contacted Norwegian authorities to express interest in the new technology, which could be used on the U.S. ships sunk in Pearl Harbor during the Japanese attack in 1941.

“The important thing is that all the warships went down between 1940 and 1945, and problems are just now starting to pop up because the steel is corroding,” said Jostein Jacobsen, a marine surveyor overseeing the sonar project for the Norwegian ship classification firm, Norske Veritas.

“Just look at Norway alone,” he continued. “There are something like 3,500 shipwrecks. There are lots of sunken warships around the world, so there’s a high potential for this system.”

The exact design is secret, but Jacobsen describes the robot sub as “box-shaped, with lots of propellers, a camera capable of taking 3-D pictures and robot arms.” The sub can dive around 2,000 feet, which its designers say is the depth where 90% of the world’s shipwrecks can be found.

Low-frequency sound waves from the 10-foot robot sub travel differently through oil and water--differences detected by a receiver on the robot when the signals bounce back.

Although sunken warships have lain on the ocean floors for decades, no technology to measure their pollution potential and locate oil has been developed before now, Jacobsen said, “because we have only focused on the environment in recent years.”

Until now, such cleanup operations have focused on randomly drilling holes to find oil, a process that experts say can dangerously weaken an unstable shipwreck and even set off ammunition inside. (That’s not considered a danger with the Bluecher, since the fire detonated much of its cache.)

Norwegian authorities estimate that examination and cleanup of the Bluecher could cost millions, but there are no plans to ask Germany to help defray the expense. “They invaded, but we sank it,” explained Sivertsen.

As it was, the sinking of the Bluecher only delayed the inevitable by a matter of hours; that evening, Oslo was in Nazi hands.

But the precious time the disaster bought Norway gave King Haakon VII time to flee Oslo and make his way to London, where he became key to the Resistance movement. And the Oslo government had enough time to spirit away gold from the Central Bank so it wouldn’t fall into enemy hands.

To the Germans, the Bluecher is a wartime tragedy. Survivor accounts tell of the confusion, desperation and poor planning that cost 600 to 1,000 men their lives, mainly because most of the servicemen could not swim, and there were few life-vests.

Third Reich newspapers would report that one Bluecher victim drowned giving the Nazi salute and that the 500 to 600 half-frozen survivors who swam 600 yards to land crawled onto the rocky shore singing “Deutschland Ueber Alles.” Survivors discredited the sensational reports.

Norway sold the rights to the Bluecher wreck to a salvager in 1950, an act German government officials condemn or approve according to convenience. One Bonn official familiar with the Bluecher case insisted that it remains German property, but in the next breath, when the cost of cleaning it up arose, immediately declared: “Norway declared itself the owner, and they can’t take it back now. They’re responsible for the wreck.

“We still oppose diving because it’s a mariners’ grave,” added the official, who spoke on condition of anonymity. “But we have nothing against it if scientists go down there to see how thick the steel is or to do research or to take steps to prevent an oil spill.”

The Germans were irked in 1990 when a Norwegian team retrieved the Bluecher’s anchor and put it on display in a national park to commemorate the 50th anniversary of the sinking.

“They risked that the oil leaks would get worse in the interests of a nationalistic mood,” the Bonn official said. “They ripped off the anchor during a critical time, under conditions that would undoubtedly rock an already unstable ship.”

Inspection of the Bluecher is expected to take “a couple of months,” Jacobsen said, and cleanup could begin early next year. If successful, he said, the new technology also could be applied to “freshly sunken” oil tankers, as well as to the 11 other wrecks that Norwegian pollution-control authorities have put on their critical list.

“We are working at the edge of what’s possible,” asserted Sivertsen. If this fails, he said, “there is no second choice.”

Finding Oil by Sonar

The Challenge: To pinpoint location of 1,200 to 1,500 tons fuel oil and assess pollution risks without unduly disturbing Bluecher wreckage.

New Technique: Mini-submarine emits low-frequency sound waves toward the wreckage. These waves penetrate the steel hull and travel differently through oil and water--differences detected by receiver on mini-sub when signals bounce back.

The Bluecher:

* 680 feet long

* Lies upside down on seabed 300 feet of water , 20 miles south of Oslo, with remains of 600 to 1,000 servicemen.

Mini-Sub:

* 10 feet long

* Propeller-driven

* Fitted with camera and robot arms

* Can dive 2,000 feet