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If you don't know what DayZ is, and you're wondering why three year-old military sim Arma 2 is currently the best-selling game on Steam, you can read all about it here . It's a game unlike anything I've ever played: freeform, terrifying, and utterly infuriating. You might have spent an hour crawling through the wilderness to get that M1014 shotgun and ALICE pack, but the bandit who just sniped you from the bushes doesn't care. He wants your beans. What's really special about DayZ, though, is how it can create memorable moments without set-pieces or scripted events. It's the players that create the drama.

1. Helicopter ambush

Helicopters aren't a common sight in DayZ. Finding one isn't the hard part – it's locating the parts to repair it. But if you ever do see one on your server, hide. The chopper, a UH-1H Huey, is equipped with side-mounted M240 machine guns. The two guys in this video are being terrorised by another player in one, and have to deal with both him and a swarm of zombies attracted by the sound of the rotors. “Come out, come out!” the pilot jeers menacingly in the chat window as his victims cower in a leaky shack.

2. Trail of zombies

The zombies in DayZ aren't just shambling piles of rotting flesh: they can run, and they'll chase you for miles if you alert them. A future patch will allow you to hide and lose any that are pursuing you, but until then you're going to have to either log out, or fight them. In this video, two guys decide to dash around like idiots and aggro as many of the undead as they can. They each get an enormous stampede charging behind them, then meet up to combine their hordes. The result is the biggest amount of sentient corpses I've ever seen in one place in DayZ.

3. Friendly fire

DayZ is an insanely tense game. You're either worrying about alerting zombies, or being sniped by some unseen player and losing all your hard-earned gear. It gets especially nightmarish when you're playing in a group, like the guys in this video. They panic when they think they see another player and have a tense stand-off, and eventual fatal shootout – only to realise they've been firing at each other. D'oh. Some servers disable certain features, including the nametags that appear when you look at other players, which makes things even more nerve-wracking.

4. Best of friends

In any great zombie movie you need a group of survivors with conflicting personalities. This is what makes DayZ Adventures , our favourite of the many YouTube series dedicated to the game, so much fun to watch. In this episode, Matt tries to hook up with with his friends who are holed up in a hospital, and brings with him – to their dismay – a massive conga line of zombies. When you're playing in a group, all it takes is one mistake or stray shot to jeopardise the entire team, which makes for some superb human drama; just like the best zombie flicks. Friendships will be tested.

5. Psychological warfare

Of course, this being an online game, you're going to get some weirdos. In this video a group of survivors are chased by another player who is repeatedly saying “No no no no no!” into his headset in a creepy voice. Even though the guys being harassed are heavily armed, and their freaky stalker isn't firing at them, they still run away in terror. DayZ is genuinely one of the most unnerving games ever. You're constantly on edge, and having someone running around making weird noises doesn't help.

6. Tractor rampage

While most vehicles and all helicopters need to be repaired before you can use them, certain vehicles spawn that are instantly usable. These include bicycles, boats, motorcycles, and tractors. There's a great location guide on the DayZ Wiki . Vehicles obviously make traversing the huge map easier, but they can also be used as weapons. In this video a guy finds a tractor and goes on a rampage to the tune of Nat King Cole's 'Unforgettable'. Tractors are fun, but they're noisy and will really stir up the horde.

7. Private security

Big groups of players often set up camp in DayZ's wilderness to regroup and store gear, and these naturally become targets for loot hungry bandits. To solve this problem, these guys allegedly hired two Chinese gold farmers to act as private security for their base of operations. This video apparently shows them in action, scouring their HQ for lurking thieves. It seems like madness to pay someone real money to protect virtual items, but stranger things have happened. Of course, it could be a hoax. You decide.

8. Pest control

DayZ is full of bugs. The game is, after all, only at the alpha testing stage. Patches have been regularly fixing glitches and exploits (here's the changelog ), but it's still full of problems. This guy spurting a fountain of blood is just the tip of the iceberg: I've seen zombies running around with no bodies, other players firing invisible guns, and one time the game spawned us under a house and I couldn't move. What's cool about playing it at this early stage is getting to see it grow and evolve. It gets better with every update, and community gripes are steadily being ironed out.

9. Good grief

Griefers. Of COURSE there are griefers in DayZ. But, in a weird way, they add to the experience. Chernarus is like a lawless Wild West, and the palpable feeling of danger really adds to the desperate, apocalyptic atmosphere. You've seen The Road, right? If society really did fall to some catastrophic virus, it would bring out the worst in humanity. In this video, two guys in choppers are strafing the coast attacking newly spawned players. Massively wanton and unfair, but hey, that's the harsh reality of DayZ. You will die, and you will be unhappy about it, but every death makes you stronger.

10. Karma's a bitch

In this video two bandits are trying to kill another survivor, and one of them gets a nasty surprise. I won't spoil the ending: just keep watching. It's not really surprising, but the majority of players in DayZ will kill you on sight. There are friendlies out there, and I've met some nice, helpful people, but I've also been stabbed in the back more times than I can remember. I approached one guy, and he saluted. I saluted back, and continued on our way. Seconds later he'd shot us in the back.