  We report the formation of a crater at the free surface of an immersed
granular bed,locally crossed by an ascending gas flow. In two dimensions, the
crater consists of two piles which develop around the location of the gas
emission. We observe that the typical size of the crater increases
logarithmically with time, independently of the gas emission dynamics. We
describe the related granular flows and give an account of the influence of the
experimental parameters, especially of the grain size and of the gas flow.
