SAN DIEGO, Calif. -- The Pentagon said it believes the greatest undersea threat facing the U.S. Navy since the end of the Cold War has arrived.

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The threat involves a new a new class of silent submarines -- subs that the U.S. Navy is having trouble finding under water. In this exclusive investigation, NBC4's Chuck Henry looked into one of those submarines in San Diego.

Following is a verbatim script from the on-air report.

CHUCK HENRY: The threat is real. And it has the Navy so concerned that it's turning to Sweden for help. That's because the Swedes have those silent submarines.

And right now, one of them is stationed at Point Loma. Earlier this month, NBC4 had unprecedented access to the Swedish sub and its crew.

What NBC4 aired few people have ever seen and certainly not in the United States.

On the surface it looks like any other submarine, but the U.S. Navy said it could be the most dangerous sub in the world.

FREDERICK LINDEN: Being noisy is something that is dangerous for us.

HENRY: Fredrick Linden is the commander of the HMS Gotland. He and his crew of 29 call the sub base at Point Loma home. They came to San Diego because the Navy is worried about this new generation of silent subs.

The Pentagon leased the Gotland for one year, but now has extended the lease for a second year, as they try to learn why this submarine so difficult to find underwater.

Subs have always had two weaknesses: they make noise and can't stay submerged very long. But the Gotland runs on a high-tech system called Air Independent Propulsion -- or AIP.

LINDEN: With AIP, I can stay submerged for weeks.

HENRY: Not only can the sub stay submerged for up to a month, the AIP technology also makes it quieter than other subs, and almost impossible to pick up on sonar.

When the Gotland wants to be silent and undetectable, especially along coastal waters, there's not a place it can't go. And that's one of the reasons it's here.

Since last summer the Navy has spent months playing a game of cat and mouse with the Gotland off San Diego, and time after time the Swedish sub has eluded its pursuers.

HENRY (TO COMMANDER): As an adversary, how good are you at what you do?

LINDEN: Very good.

HENRY: Can you tell us about your accomplishments?

LINDEN: We are satisfied with being good.

HENRY: According to Swedish newspapers, in training exercises the Gotland has sunk our most sophisticated nuclear submarines. But perhaps even more disconcerting, it reportedly sunk our largest aircraft carrier, the U.S.S. Reagan.

NORMAN POLMAR, NAVAL ANALYST: She's really run rings around our carrier groups.

HENRY: Norman Polmar is a military analyst specializing in naval intelligence. He says since the end of the cold war, funding of the navy's anti submarine warfare program has been scaled back dramatically.

As the U.S. funnels billions into the war on terror, countries like North Korea, China and Iran are building or trying to get submarines like the Gotland.

Two months ago in the Persian Gulf, Iran tested a new anti-ship missile fired by one of its subs.

If the Iranians are successful in getting a Gotland-class submarine, it could pose a new silent danger to vital oil tanker traffic in the region.

POLMAR: With more of these submarines being bought by countries that don't particularly like us, Iran being a good example, yes, there is a potential. There is a threat.

HENRY: And the commander of the Gotland knows just how vulnerable the United States could be if a sub like his fell into the wrong hands.

HENRY: If you look at our coast, North America, is there any place that you can't go?

LINDEN: No.

HENRY: No place?

Linden then shakes his head.

HENRY: Although this emerging undersea threat is a top priority for the U.S. Navy, the U.S. is committed to its nuclear submarine force, and has no plans to develop subs like the Gotland.

The Navy says it just wants to know how to detect and kill them.