Fueling operations of the SLS rocket were going well this morning until a hydrogen leak was found in the engine cavity. Propellant loading of the liquid hydrogen tank has been stopped at 8 percent full while the launch team assesses.

Why does the SLS rocket use liquid hydrogen fuel if it leaks all the time? Well, it is very efficient, energy density-wise. But most importantly, it's what space shuttle designers used 50 years ago, and Congress mandated that the SLS rocket use those same engines.

Source: @SciGuySpace