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.
 * 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 the order in which it was developed, not an indicator of its explosive power (i.e. being 1.75 more powerful than C-4).
 * It is mentioned only once when Master Chief talked about his training as a Spartan-II.
 * The most likely reason C-7 was developed was so that the users would be able to simply spray it onto the surface.
 * C-7 is another reference to 7.
 * In Halo: Fall of Reach, Samuel-034 used C-7 to destroy an airlock in order to escape from Eridanus Secundus during the capture of insurrectionist leader Colonel Robert Watts.
 * The properties of C-7 are more than likely similar to the yellow expanding foam used for insulation and sealing in construction.