A large very localised pressure perturbation has been observed in JET, due to the formation of a small region on the q = l surface with greatly increased density, which persists for a surprisingly long time. This effect, called the snake, is seen following the injection of D2 pellets, at the onset of sawtoothing and following high power neutral injection. Many detailed properties of the snakes are found, principally from soft X-ray measurements, and impurities are found to play a significant role. The snake is used as a diagnostic of the q = l surface through the sawtooth cycle. The possible mechanisms which could sustain the snake are discussed.