Abstract: A core retrieving tool for retrieving a core sample of an underground formation includes a coring assembly having a coring bit, a core barrel within a core-receiving chamber, an expandable fluid chamber in fluid communication with the core-receiving chamber, and a valve that selectively seals the core-receiving chamber. The expandable fluid chamber is expandable in response to a pressure differential between the pressure of the core-receiving chamber and an external pressure. With the valve closed, the expandable fluid chamber is in open fluid communication with the core-receiving chamber such that a pressure of the expandable fluid chamber is equalized with the pressure of the core-receiving chamber.