Out of nowhere, a new mobile game called Happy Wheels has topped Apple’s App Store in multiple countries days after its release.

Happy Wheels is a physics-based, side-scrolling game where a guy dressed in a suit avoids deadly obstacles while riding around a Segway. Its developer, Jim Bonacci, released the title on Aug. 19, where it debuted at 984th on the US App Store, according to mobile-analytics firm App Annie.

But then Happy Wheels got some serious momentum. The next day, it was at no. 10, and in the days following, it absolutely dominated the App Store. Happy Wheels has topped at least one app category, if not the country’s overall App Store, in the US, Australia, Canada, France, Germany, Italy, Japan, the Netherlands, Russia, Singapore, Spain, Sweden, Switzerland, and the UK.

It has now been the most popular app in the US for seven days and counting.

App Annie Happy Wheels iPhone daily rank history in the US (Teal is overall ranking, orange is games, yellow is action games, blue is racing games)

This is all the more impressive when you consider that Happy Wheels wasn’t even the first app that showed up when people searched for its name in the App Store—it was the 22nd result. “[S]o you’ll have to do some scrolling,” one of the Happy Wheels team members wrote in a blog post announcing the launch. “It’s exhausting, but probably worth it.”

So what was Happy Wheels’ secret to its success?

It already had a fan base

Bonacci, who did not respond to a request for an interview, first released Happy Wheels for web browsers in 2010, and he built up a fervent fan base in five years. “They did a great job of incorporating the community for user levels, user-created content,” Jeff Yates, vice president of operations at app analytics company Sensor Tower, tells Quartz.

The overnight success of Happy Wheels on the App Store illustrates the pent-up demand for a mobile version. “You have no idea how long I have been waiting for this,” wrote one reviewer who gave the app a five-star rating. (With more than 5,200 reviews, Happy Wheels has an average of 4.5 stars.) Already, players are asking when they can expect an Android version.

It marries violence and physics

This is an incredibly popular combination when it comes to mobile games, says Fabien Nicolas, App Annie’s vice president of marketing and communications. “Those games usually have between one-minute and three-minute gaming sessions,” he says. “A lot of time, males between 10 and 25 are really the core target. I don’t know what to make of it—little boys like violence and short game sessions because of ADD?”

It plays by Apple’s rules

The developer is “very savvy in being careful not setting off any warning bells with Apple by putting any overly graphic content,” says Yates. Bonacci appears to have created a tamer version of Happy Wheels for mobile (to the dismay of some users, according to the reviews), but now that the app has passed Apple’s sniff test, he will be able to add new levels—and, very likely, more gore.

“You get a spear through the throat pretty easily in the web version of the game,” Yates says. “Those things have been taken out on the first basic levels [of the mobile version]—though you can get a car dropped on you, and there is spewing blood, and you can lose a foot in the mobile version.”

If this momentum continues, Nicolas says the team behind the game “are going to make a killing” in advertising revenue. Currently, ads show up in between levels, and there are also full-screen videos and banner ads in the game. Users can choose to opt out of the ads by paying $1.99. ”If they do a good job, it’s a rags-to-riches fairy tale,” adds Nicolas. “To me, that’s what’s so unique about mobile.”