Sub Tower (Catalina — 205 fsw)

The “Sub Tower” is a classic deeper Catalina dive. It isn’t a big site and I personally believe it is under-rated, but we’ll “dive” into that…

USS Thresher

The submarine USS Thresher (SSN-593) was created to find and destroy Soviet submarines. It was the fastest and quietest submarine when she was launched on July 9th, 1960. It was an engineering feat unequaled for submarines. She was the “lead boat” in the class when she was launched and so the class of subs was originally named “Thresher.”

On April 9, 1963 at 8am, the Thresher left Kittery, Maine to conduct deep sea trials. On April 10th, during those tests, she sank 220 miles off the coast of Boston killing all 129 crew and shipboard personnel on board. There was a massive search effort that took place after the accident and the submarine debris was found. They eventually found the shattered remains of the hull about 8,400 feet deep. There is no conclusive cause for the accident and there are some interesting, yet conflicting, theories.

USS Thresher


After the USS Thresher sank, the Navy determined that it needed to take measures to maintain the safety of the US submarine fleet. It developed a comprehensive SUBSAFE program to ensure the safety of the US submarine fleet. It is a set of processes and standards that subs are required to meet. There are three “pillars” of the Navy program including Survival, Escape, and Rescue.

Before the advent of the Deep Sea Rescue Vehicle (DSRV), North American Rockwell designed and built a submersible, the Beaver Mark IV, that could dive to 2,000 feet and could mate with underwater oil capsules and other submarines. It was designed to be a work boat for undersea repairs.

Note that the picture in the upper left is the Beaver IV at the site of the future Catalina Hyperbaric Chamber.

As part of this program, they needed a universal mating flange and hatch system. Of course, once you have the system in place, you need to practice and test it. Hence, the Submarine Rescue Tower was built in 1968 to test the mating and flange system and to practice docking procedures.

The tower was largely forgotten after 1968 testing concluded. In 1990, a boat caught its anchor on the structure. At that point, the tower was still upright and moored via cables to two large, concrete blocks. The base was about 200′ deep and the top of the tower was about 130′ deep. Sometime during spring of 1993, an anchor caught and broke the tower’s rigging, causing the structure to fall perpendicular to the shoreline.

Today, the flange is about 200′ deep and the tower extends up the slope to about 175′ deep.

The Dive

Kyaa anchored the Sundiver in a protected cove (which was a bit surge-y due to the conditions and shallow water) and put down an anchor off the bow in about 55′ of water. She said “go 500 feet at 20 degrees and you should see it in 200′ of water.” Well, I’m not the best underwater navigator and Justin said he was about the same.

We got down to the anchor line and took the scooters down a ridge line. Sure enough, in about 150-170′ of water, I saw a big concrete square structure so I knew that the tower must be nearby. After stopping for a minute to look around, I saw the shadow off in the distance and headed down deeper in that direction and found it relatively easy.

We spent 15-20 minutes touring around the site and then headed back to the ridge and followed the ridge along until we saw our strobes on the anchor line at 50 feet and completed our deco on the anchor line.


I found the sub tower to be a very fun subject to photograph. The key part of the structure is the flange at the bottom and the chamber that is at that end before the tower section that climbs up the ridge.

Flange Section


Tower Sections


At some point, if I’m ever in the area in Florida, it would be cool to go see a Beaver IV at the Man in the Sea Museum in Panama City Beach, FL. It would be interesting to see if I could get a picture of the mating flange to compare it to the sub tower.

I took GoPro footage of the dive. If I have some time, I’d like to see if I can build a photogrammetry model of the tower.


Mike Wynd Video

Isthmus Deep website

Wikipedia article on USS Thresher

Subsafe Program

National Steel & Iron Museum

Cyberneticzoo Underwater Robots

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