Artist's Sketch of Caisson
Scene at Long Beach when the first completed caisson in the $690,000 Santa Monica Yacht Harbor project was successfully launched.
Image from the Los Angeles Times Mar. 12, 1933

The Caissons

Two man-made structures lie on the ocean floor just a few short miles from Los Angeles Harbor. Separated by less than a half mile, these identical structures, composed of three vertical concrete cylinders, span a distance over 100 feet and tower more than 30 feet above the seabed. "The Caissons”, as they are commonly known, are a popular destination for SCUBA divers capable of reaching their depths over 140 feet below the surface.

While they do resemble a caisson used in bridge construction work, their actual purpose has only been speculated. Were they used to protect Los Angeles Harbor during WWII? Were they used during the construction of the Vincent Thomas Bridge in the 1960’s? Are they some type of storage tanks? Or were they used as anchors for offshore oil drilling platforms? How did they get here and when were they sunk? These are some of the questions surrounding the mystery of the Caissons that have remained unanswered until now.

Artist's Vision
Image from the Los Angeles Times Dec. 13, 1931

Colorful Haven Visioned at Yacht Harbor

The caissons or “concrete cribs” were part of the original plans to construct the 2000 foot Santa Monica Yacht Harbor Breakwater in 1933. The $690,000 project called for eighteen such caissons to be placed side by side, filled with sand, and anchored to the seafloor with steel pilings. Designed by Howard B. Carter, City Engineer of Santa Monica, each steel reinforced concrete caisson, weighed 2100 tons, measured 111 feet long, thirty-five feet wide at the base, thirty-six feet high, with a diameter of twenty-eight feet at the tubular sides.

The exteriors of each unit would present curved surfaces to the forces and currents of the surging sea. The three arches on each side of the caisson would distribute the water stresses on the unit equally, exactly as arches have been used to absorb stresses equally in buildings and bridges for centuries.

Ciasson Under Construction in Long Beach
First caisson under construction.
Courtesy of the Santa Monica Public Library Image Archives

The giant caissons were built in a specially designed graving dock at the Graham Brothers’ yard in Long Beach, by the Puget Sound Bridge and Dredging Company, the general contractors for the project. Each unit was constructed with a false bottom for floating purposes.

Map of Tow Route
Image from the Los Angeles Times Mar. 12, 1933

Once built and allowed to harden for several weeks, the first caisson was floated and towed by tug under the Edison drawbridge and then the thirty miles around the coastline to its final position in Santa Monica Bay. The occasion marked the first time in the United States that an attempt had been made to float a similar concrete construction in an open sea.

Caisson Launch
Photo from the Los Angeles Times Mar. 10, 1933

Concrete Ship

Launching at Long Beach of the first crib for the Santa Monica Yacht Harbor breakwater and inset, watching the operations, Howard B. Carter, City Engineer of Santa Monica, W. F. Way, engineer in charge, and N. S. Ross, associate in the contracting firm of Puget Sound Bridge and Dredging Company and W. F. Way, Inc., of Los Angeles.

As the crib was launched it bore the name of John Morton, Commissioner of Public Works of Santa Monica. Each following unit, which were to be completed at a rate of one a week, were to be named after some beach city official.

Lining Up Caisson
Photo from the Los Angeles Times Mar. 26, 1933

Floating Breakwater Takes Trip to Home in Sea

Tugs stand by after towing caisson from Long Beach while engineer on end of Santa Monica pier lines up spot on which to sink caisson.

Caisson Close Up
Close-up of caisson being lowered into position.
Photo from the Los Angeles Times Mar. 26, 1933

Ready to Visit Davey Jones Locker

The plans for a crib type breakwater would come to an abrupt end soon after the first caisson was lowered into position. Sand scouring beneath the structure had caused the unit to crack and soon the central cylinder collapsed. The remains of the caisson were declared a menace to navigation and cleared away with dynamite. A traditional rock-mound type breakwater was eventually adopted in favor of the crib type design.

At the time of the first caissons’ failure, three completed caissons were still waiting at Long Beach for installation. Obviously this would never happen and on May 27, 1935 the remaining sections were towed out to sea and sunk. The location of the third caisson is not known, but due to the close proximity of the dump site to the San Pedro Valley, it’s possible the third caisson was sunk in deep water and is gone forever.

Navy records from 1942 indicate that the U.S.S. Gilmer made sonar contact and a subsequent depth charge attack on a stationary object on the seafloor believed at the time to be a possible enemy submarine. The range and bearing given in the report matches precisely with the location of Caisson #2. This attack would account for the severe damage to one end of the caisson that is visible today.

Caisson Side Scan Sonar Image
Side scan sonar image of Caisson #2. The far right cylinder of the caisson is completely destroyed along with half of the center cylinder. The left cylinder is intact and the two crossmembers are easily visible. The large acoustic shadow gives a good sense of the 36 foot vertical relief of the structure.

Vertical Sonar Image of Caisson
A traditional sonar recording of Caisson #2. The sheer vertical sides of the caisson are very evident as well as the debris pile from the collapsed cylinders at the base of the structure. A large population of fish make the Caissons their home.

Breakwater Coin - Front Breakwater Coin - Back
This 1933 Santa Monica Breakwater So-Called Dollar (HK-687) was struck off to commemorate the start of the breakwater project. The front of the coin (left) shows a young woman sunbathing on the beach next to the Santa Monica Pier. The back of the coin (right) depicts the completed breakwater and yacht harbor. Note the suspension bridge connecting the end of the pier to the breakwater. These coins can also be found in brass, nickel, and a rare blue aluminum version.