You have to go underground. When you killed the Makron (a joke in Danish - here it is a tiny, oversweet and easy-crunchable cookie :), run down below and shoot the new crack in the wall. Here's your secret.

When you have admired this picture for a suitable time, the floor will go boom, and you have a ladder down to...? See for yourself. Just ID's sense of humor, i guess. Oh dear.