This story originates with the mysterious sinking of the nuclear-powered attack submarine USS Thresher (SSN-593) east of Cape Cod on April 10th, 1963 and ends with a deep dive on a piece of submarine safety history off the coast of San Diego, California with a detour to Morro Bay Maritime Museum in-between.
It is a long but fascinating piece of U.S. Navy submarine safety history.
Get a large cup of coffee and settle in…
USS Thresher (SSN-593)
The sight of a nuclear-submarine cruising towards you is just so…intimidating.
The Thresher class (later renamed Permit class) nuclear-powered submarines were attack submarines that were very fast and very quiet to evade detection — but very deadly. They were designed to find and destroy Soviet submarines. They had advanced weapons systems along with passive and active sonar that could detect other vessels at unprecedented ranges.
The USS Thresher was the lead boat of the class of submarines until it sank in a tragic accident on April 10th, 1963.
The Thresher was some 220 miles off of Cape Cod, Massachusetts doing some testing and dive trials after a recent overhaul. She spent a day submerged at half of test depth and then commenced deep-dive trials. She was in touch with the submarine rescue ship Skylark during the descent but then, near the test depth of 1,300 feet, the communication was garbled and indicated that they were having “minor difficulties, have positive up-angle, attempting to blow” and another garbled message that included the number “900” and then nothing else.
The Navy mounted an extensive search operation and eventually found some debris but the sub was definitely gone, killing all 129 crew. To this day, there is a lot of uncertainty (at least in the public documents) about what happened and why she sank.
There have been many expeditions and inquiries into the sinking to find the root cause. Based on deep-sea photography, recovered artifacts, and other documents the theory is that a salt-water piping system joint failed due to silver brazing instead of welding. There is an alternate theory that it sank due to an electrical failure.
The sinking of the Thresher was a watershed event for the US Navy.
The Court of Inquiry delivered a report and recommended that the Navy implement a more rigorous submarine safety program. Part of that program, launched in December 1963, was SUBSAFE.
Submarine Safety (SUBSAFE) Program
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.
In order to effect a rescue and for the trapped sailors to escape, the Navy developed Deep Submergence Rescue Vehicles (DSRV). There was one that was “on-call” 24x7x365 and could be dispatched anywhere in the world in 24 hours or less. In order to transport the rescue sub, there was also a transport plane on-call.
The program consists of the DSRV itself, the airplane to load it into and the Mother Sub (MOSUB) that it is mated to in order to get close to the damaged sub to effect the rescue. Depending upon where in the world the stricken submarine sits, the DSRV can be on the scene anywhere from 36-48 hours after first notificaiton.
Deep Submergence Rescue Vehicles (DSRVs)
As part of the SUBSAFE, there were two DSRVs designed and built. DSRV-1 was named the Mystic and DSRV-2 was named the Avalon.
The DSRVs had a fiberglass hulls with three internal and inter-connected spheres made of high-strength steel forming the pressure hull. The front sphere is the control room for the pilot and copilot, the middle sphere and the aft sphere are used to seat the rescuees (or equipment).
Three Sphere Design
The DSRVs were about 50 feet long, 9.5 feet high, 8.2 feet wide and had 3-4 crew with up to 24 rescuees per trip and were battery operated and were capable of operating at depths up to 5,000 feet. That is simply amazing for a sub designed in the mid-1960s.
Even more amazing is how maneuverable they were.
They had to hover precisely in place which can be extremely difficult given currents and depths in the ocean. The engineers developed a special smart-vehicle system — twice as complex as the one that guided the Apollo 11 1969 moon landing — to control the thrusters and ballast system. The DSRVs could maintain a position underwater with a leeway of less than an inch. They could attach to a submarine at angles up to 45 degrees from vertical and while exposed to a current up to 2 knots.
Both of the DSRVs were stationed at North Island Naval Station in San Diego and were never required to conduct an actual rescue operation. Below is an older version of the FAA map for North Island field which shows the location of the DSRVs on the left side “C5 Deep Submergence Rescue Vehicle Loading Area.”
Below is a short documentary on the DSRV that is interesting and has that great “1970s vibe” to it.
In 2008, the DSRVs were replaced with an updated rescue system called the Submarine Rescue Diving and Recompression System (SRDRS). The Avalon was decommissioned in 2000 and is currently at the Morro Bay Maritime Museum and the Mystic was deactivated in 2008 and was donated to the Naval Undersea Museum in Keyport, Washington.
I got my degrees at Cal Poly, San Luis Obispo and I was back in the area a couple weeks ago.
I had contacted the Morro Bay Maritime Museum in advance to see if they would let me photograph the Avalon before the general public showed up and they were very gracious and allowed me to do that. I took a number of pictures and also built a photogrammetry model. Some of those pictures and a link to the model are below.
All of that brings us to the actual dive site.
Back when they built the DSRV, they needed a way to test the system and perform routine training exercises. They built two fixtures off of San Diego in order to conduct training and testing.
All of the rescue assets, the Deep Submergence Rescue Vehicle system and Submarine Rescue Chamber system, routinely exercise in the waters off Southern California to maintain training proficiency and verify readiness. Two pre-positioned fixtures, simulating submarine hatches, are mounted to the sea floor and are used to support the DSRV. One unit, Deep Seat, is located at a depth of 2000 ft. The second training fixture, Deep Throne, is located at a depth of 250 ft and consists of mating surfaces at angles up to 40 degrees. In addition, these rescue assets exercise with U.S. submarines and participate in international rescue exercises.GlobalSecurity.org
Tyler had first mentioned Deep Throne to me maybe 18-24 months ago and it has been on my list for a long time.
In order to safely conduct a 300 foot dive in the Pacific Ocean off Southern California, a lot of pieces have to fall into place. The first time I was going to dive it was with Ben Lair and Justin Judd sometime in 2021 and I just wasn’t feeling like doing a 300 foot dive so we went on a different dive.
The second time I went to dive it with Drew Wilson on Feb 1, 2022. Tyler and others had dove the site a long time ago but the coordinates were lost so we had to take a guess at the location. Based on the bathymetry data, we went to the possible location. After doing a quick look at the sonar, we thought we had it nailed. Drew and I suited up, got all of our gear on and went down the line and found … a shelf 20+ feet above the ocean floor! I should have paid closer attention to the sonar image and realized it wasn’t the right height or structure. Oh well. Below are Drew’s photo of the shelf (I didn’t even turn on my camera) and the sonar image.
After the dive and deco, we started looking around the obstruction area on the NOAA charts and we were pretty sure that we had located it but weren’t ready to do another 300 foot dive so we went and dove the 210′ deep prop and engine of the B-36 that I recently wrote about.
We rescheduled our return dive for Monday of President’s Day weekend but got foiled by bad weather and had to call it off. We then rescheduled for March 2nd and finally got to dive the real site!
The location of the DeepThrone structure is about 4.5 miles offshore. There is an obstruction marker on the NOAA charts in 41 fathoms (246′) of water which would be about the depth of the top of the structure. However, we found that location to be slightly off of the actual location of 32 47.218 N, 117 21.330 W. Below are snippets of the NOOA map, the Google map of our dive location along with an image of the massive sonar return from the structure.
We had both been diving recently and knew that the surface layer had a lot of particulate matter and that the visibility was poor above 100+ feet or so. Sure enough, we got below the “muck layer” and the visibility improved greatly; however, due that layer of “muck” there wasn’t much light as we descended. We were both expecting to see the structure at about 250 feet but didn’t see anything and my heart started to sink.
Then, we reached the bottom of the downline and the structure was straight in front of us! It was very, very dark at depth. We had planned to do our “standard” plan which is that I take photos using the powerful and bright Keldan video lights while Drew scooters off into the blue and takes pictures of me against the structure. After about 12.5 minutes of bottom time, my @+5 was 25 minutes (an extra 5 minutes at the bottom would “cost” me 25 more minutes of decompression) and I signaled to Drew that it was time to go.
Somehow, I had got turned around and didn’t see the downline (that never happens to me) and wasn’t sure if Drew had put a strobe on it. Drew fortunately had a good idea of where it was and did a short scooter run to verify his strobe and then came back and towed me to the line and we started our 90 minute decompression profile
Deep Throne Photos
Structure / Shape
The structure itself is an interesting shape.
I was expecting a “dome” based on some video that a friend of Tyler’s had shot when they originally dived it. It is a little hard to describe the structure. It has four “legs” that look to me like the base of the Eiffel Tower, but the structure is rectangular and not square. There are a lot of support crossbeams and then a rounded plate that Drew and I described as a “inverted skateboard ramp” across the top and has the three test docking locations. However, they appear to be covered up by plates that have been welded on. I’m not sure why that is.
There are also a TON of white metridiums on the structure. They are really amazing.
Here are some photos that show the overall structure. Given the dark conditions, it would have been impossible to get a photo of the overall structure at that depth and I didn’t have enough time to shoot enough photos for a complete photogrammetry model (although that would have been very cool).
Below is a photo from Drew that I have taken the liberty of increasing exposure post-production which has resulted in more grain but it gives a good sense of the base and the curved plate I mentioned:
Below is a similar photo that shows more of the curved plate that has the docking stations. This photo also gives a better sense of the scale of the site.
Below are some photos I took of the structure with information in the captions. A lot of the photos look to me like they are from an outer space movie given the blackness of the background and the curvature of the structure.
As always, Lora & Chris at Marissa Charters. Supporting divers on 300′ dives in the Pacific Ocean is tricky and they always do a great job and get the downline perfect. You can see how close it was in a couple of the photos!
Thanks to Drew Wilson for being a great dive partner and helping with photographs.
Thanks to Tyler Stalter for, well, being Tyler and always inspiring me to dive new and interesting sites. I would probably have never known about Deep Throne without him.
I tried to list them all. There were a LOT of on-line sites I used when researching and writing this article.