115 originated as a hidden musical Easter egg in Treyarch's Call of Duty: Black Ops Zombies mode, first heard on the Kino der Toten map in 2010 and performed by Elena Siegman. In the game's lore, 115 refers to Element 115, the mysterious substance behind the zombie outbreak, which gave the track its eerie, otherworldly feel. Players discovered it by activating radios around the map, and clips soon spread through gameplay videos and montages on YouTube, where it became shorthand for the classic Zombies atmosphere and nostalgia for the early Black Ops era.