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, 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