Turret Opera

The Turret Opera is an event in the single-player campaign of Portal 2 in which a group of Sentry Turrets form a unique melody via the movement sounds of their weaponry casings.

The opera is introduced as an easter egg in Test Chamber 16 of GLaDOS' testing tracks in Chapter 3: The Return, hidden beside a wall with a gap near the start of the chamber, where a Turret can be seen inside it. Upon entering, Chell would be able to witness various Turrets beneath the room as they serenade to the Prima Donna Turret.

Trivia

 * The word "ciel" in the lyrics is a play on words; it is pronounced almost the same as Chell.
 * Before becoming a voice actress, Ellen McLain was an opera singer.
 * It was revealed by Ellen McLain that she had done both Want You Gone and the Turret Opera finale in just a two hour leap after finishing one of them.
 * In Portal 2, Weighted Companion Cubes can be heard humming it when nearby.
 * It was revealed by Ellen McLain that she had done both Want You Gone and the Turret Opera finale in just a two hour leap after finishing one of them.
 * In Portal 2, Weighted Companion Cubes can be heard humming it when nearby.

Sheet Music
Sebastian Wolff has transcribed the song and provided free sheet music: "Cara Mia Addio" can also be found on Spotify.
 * Sheet music for choir (SATB)
 * Sheet music for solo piano