C-12 shaped charge

The UNSC C-12 Shaped-charges or C-12 SCs are used mainly for heavy demolitions and can also be used as weapons. This compound is extremely dangerous and very volatile. A small 4x4x4 inch cube alone can level a five story building. It's solid, but malleable enough where it can be shaped or bent into different forms to stick to surfaces better and is detonated with electro-shock detonator sticks or remote detonators.

C-12 is also hefted in medium sized backpacks called "Damage Packs" or "Blow Packs". Two Damage Packs contain enough C-12 to blow through three meters of UNSC standard armor plating.

During the first assault of the rebels on the Erandinus System, which happened to be the Spartan's first real mission, Sam hefted two medium-sized backpacks full of these. After capturing the leader, Watts, Sam used these to blow clean through the base’s pressure doors. However, in doing so, he also murdered uncountable "civilians", or non-soldier sympathizers.

Trivia

 * During World War II there was a reasonably powerful explosive called C-2, which is a precursor to the present day C-4 and possibly to C-12.