Experiments were performed using ultrashort (30fs) ultraintense (up to 1018 W/cm2) laser pulse focused into underdense deuterium plasmas (ne＜1020/cm3). Macroscopic bubblelike structures were observed using monochromatic plasma irradiance imaging method. These structures are interpreted as the overdense shells of postsolitons generated in the plasma by the laser pulse, which is found to have important influence on the formation of plasma channel.