Liverpool: You'll Never Walk Alone. West Ham: I'm Forever Blowing Bubbles. Dortmund: Heja BVB. AC Milan: the Champions League anthem (because they've heard it 200+ times). The walk-out song IS the club.
In Germany, fans hold scarves above their heads during the anthem. In Turkey, fans create a wall of sound so loud you can't hear yourself think. In Argentina, the Bombonera literally shakes. Each culture celebrates differently.
Banned almost everywhere. Used almost everywhere. Greek ultras, Polish ultras, and Balkan fans turn stadiums into infernos. Dangerous? Yes. Spectacular? Also yes. UEFA fines clubs millions. Nobody cares.
Iceland popularized it at Euro 2016. 30,000 people clapping in unison, getting faster and faster. Now every team does it. Atletico Madrid do it after wins. Crystal Palace do it. It's the most copied fan tradition of the decade.