Under intense pressure hydrogen transitions into a metallic state. Under normal condition when that pressure is eased it reverts back to a liquid or gaseous state. However, foam-phase munitions were designed to allow a mass of metallic hydrogen to remain stable, until impact, where it would explode catastrophically. Foam-phase weapons could be used conventionally, or as triggers to larger fusion/fission bombs.
Foam-phase weaponry became a staple of the Demarchist military after the Melding Plague, when they lost the ability to produce antimatter in militarily useful quantities. Gas giants, such as Tangerine Dream, naturally contained metallic hydrogen at extreme depths.