Being stuck in a giant flat surface with no other job than to run around and shoot, the craziness periodically rises as the enemy horde grows into uncountable numbers, several factions stacking one atop the other, all trying to give you a hug.
Gunmen sit at the heart of these giant robots and pummel uncountable amounts of ammunition into an even greater number of octopus - like aliens that are raining through the rock ceiling.