Waymo, the self-driving car project launched by Google, has been at the forefront of developing autonomous technology, having rolled out while to deploy robotcars on the road. Part of the company’s effort development efforts also relies upon testing autonomous cars in a virtual world created by one of the company’s engineers, .

The virtual world exists in a corner of Google parent company Alphabet’s campus in California. And as part of an exclusive peek of how the virtual world works, The Atlantic learned the software is called Carcraft, a nod to World of Warcraft.

The Atlantic’s piece is a massive undertaking that offers new insight into how Waymo is working on self-driving cars, but the Carcraft world is one of the most interesting revelations.

An exclusive look at how Alphabet understands its most ambitious artificial intelligence project

Here’s a portion of the story:

Originally developed as a way to “play back” scenes that the cars experienced while driving on public roads, Carcraft, and simulation generally, have taken on an ever-larger role within the self-driving program.

At any time, there are now 25,000 virtual self-driving cars making their way through fully modeled versions of Austin, Mountain View, and Phoenix, as well as test-track scenarios. Waymo might simulate driving down a particularly tricky road hundreds of thousands of times in a single day. Collectively, they now drive 8 million miles per day in the virtual world. In 2016, they logged 2.5 billion virtual miles versus a little over 3 million miles by Google’s IRL self-driving cars that run on public roads. And crucially, the virtual miles focus on what Waymo people invariably call “interesting” miles in which they might learn something new. These are not boring highway commuter miles.

The entire story’s worth a read, and unless competitors have also kept hidden massive robotcar worlds, it shows Waymo .