What happens to helium below about two kelvin?

It becomes a superfluid, with zero viscosity - and the consequences look like a violation of common sense. It flows through gaps too small for any ordinary liquid, and it climbs: a film a few atoms thick creeps up the inside wall of a container, over the rim, and drips off the bottom until the vessel is empty.

It also conducts heat so efficiently that it cannot boil. An ordinary liquid boils by forming bubbles at hot spots; superfluid helium equalises temperature faster than a hot spot can form, so the surface simply evaporates and the whole thing goes eerily still at the transition - which is how experimenters know it has happened, by watching the bubbling stop.

The reason is quantum mechanics operating at a scale you can see. Below the lambda point a large fraction of the atoms occupy the same quantum state and behave as a single entity, so the liquid stops having the internal friction that viscosity is. It is one of very few places where quantum behaviour is directly visible rather than inferred, and it is the same physics underlying superconductivity - which is why it took several Nobel Prizes to explain.