On May 23, 1939, the submarine USS Squalus sank off the coast of New Hampshire killing 29 sailors. But 33 others survived in what is often called the greatest underwater rescue of all time.

The anniversary of the tragedy that was the sinking of the USS Thresher on April 10, 1963, just passed. All 129 of its crew was lost.

That is a terrible story, but many people don't know the story of another incident involving a submarine that was built at the Portsmouth Naval Shipyard that might have suffered a similar fate but not for some heroic efforts.

The USS Squalus was a Sargo-class submarine.

While on a test dive off the New Hampshire, it went down. What transpired over the next 23 hours is an incredible story of heroism and survival.

It all took place 13 miles southeast of Portsmouth, near the Isles of Shoals.

Do yourself a favor and watch the video, as well as take time to read the entire story found at the link below. It's from the New England Historical Society, and well worth the read.

I guarantee you'll think of the USS Squalus and the rescue effort each time you gaze out at the Shoals from shore.

It just might be the greatest underwater rescue of all time.