Liquid helium is unusually hard to store.
Helium oozes through the pores, and provides refrigeration by instantly evaporating, just like sweating in a desert climate. Slosh baffles help suppress tidal motions in the liquid. The dewar operates at low pressures, making the helium superfluid. Superfluidity is a peculiar state of liquid helium at temperatures below 2.16K where, under certain conditions, it flows without viscous resistance and transports heat far better than any metal.