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
It's 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.
 * It is mentioned only once when Master Chief talked about his training as a Spartan