A method of storing biological samples in which the biological samples are stored in a stack within a vessel interior of an insulated vessel. A pool of liquid cryogen is maintained within the vessel interior by condensing cryogenic vapor boiled off of the pool of liquid cryogen with a cryocooler having a cold finger. The cryocooler and cold finger are positioned so that a cold heat exchanger of the cold finger is in thermal contact with the cryogenic vapor and is positioned above the stack, so as to also maintain a temperature difference within the insulated vessel, between a bottom region of the stack and a top region of the stack, of no greater than about 65K.