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 shock administered through a detonator stick. 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 precuser