Disclosed herein is a bellow for use in a ball valve, the bellow comprising: a metallic seat at an end of the bellow for providing a contact surface with a ball of a ball valve; and a metallic body integral with the seat; wherein the metallic body has a longitudinally elastic property such that, when the bellow is in use in a ball valve and the bellow is arranged within the housing of the ball valve so that it is always under longitudinal compression, substantially only the compression in the bellow causes the seat to apply a sealing force against the ball of the ball valve that maintains a mechanical seal between the seat and the ball valve. Advantageously, embodiments allow a new design of ball valve that maintains a high standard of performance in subsea environments over long time periods.