MRL
From Halopedia, the Halo wiki
Summary
File:MC Boot.JPG
With the boot lightened, the egg becomes quite apparent.
There 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).
