Abstract: A system is described which enables the safe release of formation gases when corebarrel is being raised. A corebarrel has an elongate cylindrical outer casing (1) and an elongate cylindrical liner (2), the liner (2) being disposed within the outer casing (1) such that there is a space (3) between the liner and the outer casing. The liner (2) has a plurality of unidirectional valves (4) which enable the passage of gas from the volume (5) within the liner (2) to the space (3) between the liner and outer casing, and prevent the passage of fluids from the space (3) to the volume (5).