This article originally appeared in Xbox World magazine.
Played every hour of the Call of Duty series? Think you've seen it all? Think again, soldier. Over the next three pages, we list the secrets hidden in the world's biggest franchise...
1. The Quake connection
Since Call of Duty 2, the series has used the IW Engine. This began life as a heavily modified version of the Quake 3 engine - under license from legendary FPS pioneers id Software. The engine was also used for the Quantum of Solace spin-off game.
2. Rock and roll
In Call of Duty: Finest Hour, Sergeant Starkey - who we met during the attack on a German base in Tunisia - was voiced by geordie AC/DC lead singer Brian Johnson. His father was a Sergeant Major in the British Army, and he based his voice on him.
3. Unused content
PC modders digging deeply through the files of a wide number of the Call of Duty games have found references to, among other things, an unused mission set aboard the International Space Station in Modern Warfare 2.
4. Striving for authenticity
Always striving to add the upmost authenticity to their games, while they were developing Call of Duty 4, the developers attended a live fire training session at the Twentynine Palms military base in California. They stood next to Abrams tanks as they fired so they could recreate the sensation of both noise and vibration in the game.
5. Time traveller
Captain Price, who has since become one of the CoD's most loved characters, appeared in both Call of Duty 2, set in 1940s WW2, and Call of Duty 4, set in the modern day. Any explanation has been left ambiguous. A distant relative?