Turret Quartet

The Turret Quartet is an easter egg in Portal 2.

Location
In the first test where turrets can be destroyed with Thermal Discouragement Beams, there is a turret behind a metal grating. If this turret is destroyed, the grating will come off, and the player can then enter the vent and discover four singing turrets. They are all facing a fat turret opposite them, which is not singing.

Overview
Below the vent are four singing turrets, each in a separate compartment and with an idle fat turret opposite them. It is possible that these dour turrets are the same four that are seen in the ending in the elevator ride to the surface, and that the fat turret may be the same one from the ending opera. Also inside the vent are scribblings presumably done by Doug Ratmann.