Under extreme pressures and high enough temperatures – such as in the cores of Jupiter, Saturn, Uranus, and Neptune – hydrogen enters a strange phase
Small niggle here; Uranus and Neptune are too small to have metallic hydrogen in their cores - the pressures are simply not high enough. Saturn is probably large enough for a small metallic hydrogen core, although some models say otherwise.
It should also be mentioned that metallic hydrogen has been created on Earth using a diamond anvil, albeit for only a few milliseconds. However the continuous state change from a non-metallic to a metallic state has already been observed.