1. Jump around
The very best secret RC-XD track is on NukeTown, where a dirt-track goes outside of the usual confines of the map. Boost for the jump halfway round and you'll be causing a ruckus at the enemy spawn point in no time...
2. Code breaker
Towards the end of the game, if you pause during the Revelations and Redemption missions you'll see your objectives have been replaced by a familiar code...
3. Freedom fighter
You can break free from your shackles on the menu screen. Look down and then play with the trigger buttons: soon you'll be a free man with a 15G Achievement. Not that there's much to do in your dingy prison. Or is there?
To get a sonic uber-weapon, kill the bad guys with grenades after the 'eat-glass-smash-mouth' scene in Kowloon and a nearby video recorder will open. Take the tape, then hold 'use' on the similar machine in the secret armoury later on. Et voila: twelve shots of sonic glory.
5. Get to the point
On MP maps there are countless vault points you discover through research and experimentation. Our favourite? On Summit: the dual-layer radio mast you get to by vaulting up to the gantry from the computer banks and the swaying cable car.
6. Play time
When we said there's not much to do when you escape on the menu screen, we lied. Find a computer, use it, then tap in the letters DOA. A secret top-down shooter called Dead Ops Arcade will be unlocked.
7. Four-letter word
Actually, we lied twice. For an extra 15G, type 'Zork' into the computer terminal and you'll boot up the classic text adventure of the same name. Also within the mainframe are secret emails and concept art.
8. Matter of time
There are some neat little touches to many of the MP maps. Check the clock in the NW buildings of Cracked - it'll show the real time where you are. Also, the mailbox names in NukeTown are quite familiar...
9. Heads will roll
You'll need a bunch of people who won't kill each other for a while, but knock off every mannequin head in NukeTown in a speedy fashion and the Rolling Stones' Gimme Shelter will serenade your killing spree...
10. Rock solid
In the Kino Der Toten Zombies map there are three meteor fragments - one in some scattered metal letters, another behind some dummies and the other by the main stairs. 'Use' them all, and a secret song will play: 115 by Elena Siegman.