VX 7 Nerve Gas

Summary
VX 7 nerve gas is a highly toxic nerve agent used by the UNSC. Proof that it was at least planned to be used in engagements with the Covenant can be seen on the small crates, labeled "VX - 7," scattered throughout Cairo Station and throughout New Mombasa and Old Mombasa.

Trivia
VX 7. Seven is a number often used in Bungie games.