C-7 foaming explosive

C-7 foaming explosive is a highly explosive compound that is used in demolitions by UNSC forces. It is comprised of various chemicals that react to a signal electrical current shock sent through a detonation switch. It is liquid in vacuum, but once contact with a hard surface and oxygen occurs, it becomes a foaming, semi-solid adhesive resin.

Uses
Its most notable use is when Corporal Locklear used it to destroy the Forerunner Crystal (the reason why the Covenant were chasing them), leading to his death.

Trivia
C-4 is short for "Composition 4", therefore it is likely that C-7 means "Composition 7." It is also likely that it is simply a codename based on the order in which it was developed, not an indicator of its explosive power.

List of appearances

 * Halo: The Fall of Reach
 * Halo: First Strike
 * Halo: Fall of Reach
 * Invasion