You might be thinking of helium, which becomes a zero viscosity superfluid when cold enough.

Hydrogen can diffuse through certain metals and weaken them [1]. Currently, hydrogen storage is costly and tricky for a number of reasons [2]. It’s also very important to avoid leaks into enclosed areas since it’s extremely flammable.

[1]: https://en.wikipedia.org/wiki/Hydrogen_embrittlement

[2]: https://www.energy.gov/eere/fuelcells/hydrogen-storage-chall...

Oh, didn't know it'd leach into metals, that's interesting.