Caisson #2 (San Pedro, 160 fsw)

Cover Photo of me at Caisson #2 by Ben Liar

There are interesting (and somewhat funny) stories about the “Caissons” that are outside San Pedro harbor.

First, to dispel a possible misunderstanding, these are not “real” caissons (definition: a large watertight chamber, open at the bottom, from which the water is kept out by air pressure and in which construction work may be carried out under water). These were never used under water for divers and are only visited by divers nowadays.

The excellent ub88.org site has a lot of history of the caissons so I won’t repeat the entirety of their research here but will instead just give a summary. The caissons were originally designed to be used to build a breakwater in Santa Monica. They consisted of three, connected semi-circles of concrete that were 111 feet long, 36 feet high, and 35 feet wide with a diameter of 28 feet.

Constructing the first caisson in Long Beach

They pre-built 4 of the structures. The first one was floated and towed thirty miles up to Santa Monica. It was put into position but the sand scoured the bottom of the structure and caused the unit to crack and then the central cylinder collapsed. That was the end of that.

The remaining three caissons were towed out to sea and sunk. Two of them have been found. It is likely that the third was a little further out and feel into the deep sea trench.

One of the two caissons (the deeper one that we dived) only has “one-and-a-half” of the three cylinders. The “half” cylinder walls can be seen in the photos below:

The story of what happened is funny and interesting. After the Japanese attacked Pearl Harbor, we went into high alert for enemy submarines on the west coast of the United States and had regular patrols. In 1942, the USS Gilmer made sonar contact with a stationary object and thought it was an enemy submarine. They proceeded to drop 8 depth charges and blowing up half of the caisson!

This particular caisson is fun to dive. There are many “nooks and crannies” due to the explosion and resulting damage. There are small entry points that allow you to get into and out of the inside section of the caisson and explore around.

I highly recommend diving the caissons if you get a chance. Below are some photos. I was using my new camera (Sony a6400 with the Nauticam housing and WWL-C lens) and let the ISO get too high so some of the photos are a bit grainy.

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