Zwift, a 350-person, Long Beach, California-based online fitness platform that immerses cyclists and runners in 3D-generated worlds, just raised a hefty $450 million in funding led by the investment firm KKR in exchange for a minority stake in its business.

Permira,  the Amazon Alexa Fund and Specialized Bicycle’s venture capital fund, Zone 5 Ventures, also joined the round, alongside earlier backers Highland Europe, Novator, Causeway Media and True, which is a Europe-based consumer specialist firm.

Zwift  has now raised $620 million altogether and is valued at north of $1 billion.

Why such a big round? Right now, the company just makes an app, albeit a popular one.

Since its 2015 founding, 2.5 million people have signed up to enter a world that, as Outside magazine once described it, is “part social-media platform, part personal trainer, part computer game.” That particular combination makes Zwift’s app appealing to both recreational riders and pros looking to train no matter the conditions outside.