MRL

With the boot lightened, the egg becomes quite apparent.

The Initials on Master Chief's Boots are three letters ("MRL") hidden on the bottom tread of the Master Chief's boot on Halo: Combat Evolved. They are, indeed, initials - those of Marcus R. Lehto, Halo's Art Director (and the man behind the Master Chief's skin and armor design). An easy way to see the initials is if you're on a reflective surface (e.g: on Two Betrayals, while on the Control Room's bridge) just look down on the floor and on Master Chief's boot are the initials.