For this achievement you will need to load up the map Coagulation on Halo 2 Classic, this can't be done on anniversary edition.

You will want to start near the red base. Just outside the base you should see a formation of three rocks.

In the formation on the rock closest to the cliff side, you should see a face in the rock. Look at it for a few seconds and the achievement will unlock!

The same rock formation can be found outside blue base, but that side will not unlock the achievement. It has to be on the red team side of the map.