Basics

The liquid state of hydrogen
Useful as a fuel storage method, and as a Cryogen
Often Abbreviated as LH2
According to wikipedia "To exist as a liquid, H2 must be cooled below its critical point of 33 K. However, for it to be in a fully liquid state without boiling at atmospheric pressure, H2 needs to be cooled to 20.28 K[3] (−423.17 °F; −252.87 °C)."
A Common Rocket Fuel (Usually with LOX )
Neutron Scattering Supposedly Uses LH2
LH2 is also used for particle physics with Bubble Chambers although they have largely been replacecdd by other technologies

Advantages

It's a Cryogenic Liquid (thus can burn people), and requires intense cooling
Cryogenic Liquid Boil-Off caused losses, or required active cooling
Asyphxiation Hazard (although not as bad as heavier gasses, in large enough volumes, it can displace oxygen, thus ventillation and/or sensors are needed
As with any hydrogen, a pure flame is invisible in the visible spectrum, and thus may be harder to detect
As spacex's starship tests have shown, poorly welded/constructed tanks have a nasty habit of bursting and/or exploding

The Wikipedia Page on Liquid Hydrogen
Archival Footage of 1960's Testing on Liquid Hydrogen Storage Hazards by the USA Air Force (shows leaks, and fires, and compares them to similar JP-4, and gasoline etc events) (Didn't detonate in small leaks (1+1/4 gal), only a slow flame (still a flame, but not an explosion) ,similar for 32 gallon (albeit more flame), All in all, without pre-mixing detonation does not happen, in premixed, little to no overpressure was observed, but the thermal radiation may start fires
- All in all less bad than an ignition event of liquid hydrocarbons (If not mixed with air/oxygen, if ignighted NOT by an explosive, etc), but is easier to ignite
- Granted this is OLD, and the main measurements were with film? NEEDS MORE RESEARCh