The stability moment of a ship is the product of the righting lever and the total weight. In head or following seas, the righting lever varies periodically due to the changing wave elevation around the ship and her pitch motion. This, in turn, causes the stability moment to vary, which can trigger rolling. The phenomenon is known as parametric rolling because its source is the time variation of a parameter. This resonance can cause the ship to roll to very large angles in a moderate sea, leading to cargo damage, loss of containers and, in extreme cases, capsizing of the ship.
Ships capsized although they fulfilled the criteria of stability. Analysis of some cases highlighted the parametric resonance as the cause of the disaster. The surprising discovery was that the righting arm could be negative on wave crest.