Helium is actually pretty hard to keep ahold of, being a very light and small noble gas. It can diffuse through a surprising amount of materials, flow through far smaller cracks than you would expect, and is quite hard to filter out of a mixture of gases.

Also superfluid helium (a big chunk of helium used for refrigeration like in e.g. the LHC) has the weird property of flowing the same speed through a tiny hole as a large one and coating everything with a molecular coating. Superfluid helium is basically a bose einstein condensate but macro-scale, totally counterintuitive. Essentially a thermal superconductor. Zero viscosity.