C-7 foaming explosive

C-7 Foaming Spray 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 uses it to destroy the Forerunner Crystal (the reason why the Covenant were chasing them), leading to his death.

Trivia

 * In the present, there is a very effective explosive called C-4, possibly a precursor of C-7.
 * It is mentioned only once when Master Chief talked about his training as a Spartan.
 * The most likely reason C-7 was developed was so that, instead of lugging around big, blocky charges, the users would be able, to simply spray it onto the surface.
 * C-7 may be another reference to 7.