In 2001 a storm induced the failure of four caissons recently built for the construction of a dyke in a new entrance to the Barcelona Harbor. Understanding the failure required the analysis of the construction history and the wave action. The caissons were founded on a thick layer of soft silty clays potentially liquefactable. The main properties of foundation soils are discussed based on laboratory and field data. A back-analysis of the failure is developed using an analytical and a numerical procedure. The analytical procedure is based on well-known solutions and concepts in Soil Mechanics. It involves the calculation of pore water pressure distributions on the foundation, its dissipation, the induced increase of the undrained strength, and the caisson stability during construction stages. An analysis of liquefaction, which explains the collapse observed, is also described. The problem is also solved numerically using a commercial finite difference code. This second analysis allowed a cross-checking of the analytical solution and the quantification of some simplifications introduced that can be properly accounted for in the numerical analysis. In the final part of the paper an alternative stable design of the caisson dyke is presented.