You were browsing through old Serious Sam forums to find some good mods to play around with, when an old, and now closed, branch caught your eye. It told about some creepy (or crappy by other half of commenters) mod, that had a few levels tied to each other as "floors". You browsed through it to find a download link, and, gladly, it was still up and working. And now, after loading the .gro file of the mod, it was successfully added to the menu. All left to do is to press "Start".