This guide will let you know how to get under the map anywhere in Halo Infinite. You have to follow the simple steps in this guide to get under the map.
You are required to have the first strike grapple upgrade and access to a Scorpio tank to do this. You will need to crouch behind the scorpion, fire the grapple at the ground below it, and hold the melee to activate the strike.
You have to be careful, as it can be somewhat janky, and you can even land in areas with no ground.
It is a very reliable method; there could even be easter eggs tucked away under the map for avid hunters.
